On main: IMP-09 PR2 sketch (Stage 2 design reference; re-derive per unit)

Stage 3 round 4 lock implementation: extend build_layout_css beyond
the horizontal-2-only dynamic path. Every layout_css return now
carries length-locked col-axis keys (widths_px, width_ratios,
dynamic_cols) matching the parsed css_areas grid (R rows, C cols),
so 2-D layouts (T / 2x2 in PR 2) and the unified
_compute_per_zone_geometry mapper can plug in without further
contract churn.

PR 1 scope:
  - _parse_css_areas + _parse_fr_string + _compute_per_zone_geometry
    (unified — 1-D and 2-D from the same code path)
  - compute_zone_layout_cols (vertical-2 weight-only solver)
  - _build_fr_default / _build_rows_dynamic / _build_cols_dynamic
    (populate widths_px/heights_px on every return path)
  - build_layout_css override branch keeps the warn-and-fallthrough
    legacy for unsupported presets (PR 2 promotes to strict raise)
  - retry gate in _attempt_zone_ratio_retry skips when dynamic_cols=True
    or dynamic_rows=False, with explicit retry_skipped_reason
  - Step 8 artifact gains zone_widths_px_planned /
    zone_col_ratios_planned (top-level) + zone_width_px_planned /
    zone_col_ratio_planned (per-zone)
  - debug_zones width injection via _compute_per_zone_geometry
    (replaces the legacy row-only zip)

Tests: tests/phase_z2/ — 47 new cases (parse / fr-string / cols solver /
per-zone geometry / build_layout_css contract / retry gate +
6 build_layout_css YAML fixtures + 3 retry_gate fixtures).

Verification: python -m pytest -q tests = 89 passed (was 42).
horizontal-2 grid CSS strings (areas/cols/rows) byte-identical to
legacy; only additive col-axis keys are introduced.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Front/client/src/pages/Home.tsx

@@ -300,6 +300,35 @@ export default function Home() {
       if (zoneGeometries && Object.keys(zoneGeometries).length > 0) {
         overrides.zoneGeometries = zoneGeometries;
       }
+
+      // IMP-08 B-3 : zoneSections forward only when the user diverged from
+      // the auto plan. Codex Stage 3 R3 B3 fix : `createInitialUserSelection`
+      // seeds `zone_sections` with the default placement, so a literal copy
+      // would pollute backend assignment-source provenance even on a fresh
+      // re-render. Diff against `sourcePlan.zones[].section_ids` per zone and
+      // only emit zones whose section list differs.
+      const userZoneSections = state.userSelection.overrides.zone_sections;
+      if (userZoneSections) {
+        const defaultByZone = new Map<string, string[]>();
+        sourcePlan.zones.forEach((z) => {
+          defaultByZone.set(z.zone_id, z.section_ids);
+        });
+        const zoneSectionsDiff: Record<string, string[]> = {};
+        for (const [zoneId, sids] of Object.entries(userZoneSections)) {
+          if (!Array.isArray(sids)) continue;
+          const cleaned = sids.filter((s) => typeof s === "string" && s.trim());
+          const defaults = defaultByZone.get(zoneId) ?? [];
+          const sameAsDefault =
+            cleaned.length === defaults.length &&
+            cleaned.every((sid, i) => sid === defaults[i]);
+          if (!sameAsDefault) {
+            zoneSectionsDiff[zoneId] = cleaned;
+          }
+        }
+        if (Object.keys(zoneSectionsDiff).length > 0) {
+          overrides.zoneSections = zoneSectionsDiff;
+        }
+      }
     }
 
     setState((p) => ({ ...p, isLoading: true }));
@@ -310,6 +339,8 @@ export default function Home() {
         ? `(overrides: ${[
             overrides.layout && `layout=${overrides.layout}`,
             overrides.frames && `frames=${Object.keys(overrides.frames).length}`,
+            overrides.zoneSections &&
+              `zoneSections=${Object.keys(overrides.zoneSections).length}`,
           ]
             .filter(Boolean)
             .join(", ")})`

Front/client/src/services/designAgentApi.ts

@@ -251,6 +251,11 @@ export interface PipelineOverrides {
   /** zone_id (top/bottom/left/right/...) → slide-body 내부 0~1 비율.
    *  backend 의 build_layout_css 가 horizontal-2 / vertical-2 만 처리. */
   zoneGeometries?: Record<string, { x: number; y: number; w: number; h: number }>;
+  /** IMP-08 B-3 : zone_id -> list of section_id assignments
+   *  (canonical ordinal `${parent}-sub-${n}`). Only forwarded when the
+   *  user explicitly diverges from the auto plan; default placements
+   *  are not echoed back to avoid polluting override provenance. */
+  zoneSections?: Record<string, string[]>;
 }
 
 export async function runPipeline(

Front/vite.config.ts

@@ -241,6 +241,9 @@ function vitePluginPhaseZApi(): Plugin {
               layout?: string;
               frames?: Record<string, string>; // unit_id → template_id
               zoneGeometries?: Record<string, { x: number; y: number; w: number; h: number }>; // zone_id → bbox (slide-body 내부 0~1)
+              // IMP-08 B-3 : zone_id -> list of canonical section_id assignments
+              // (e.g., "top": ["03-1-sub-1"]). Forwarded as --override-section-assignment.
+              zoneSections?: Record<string, string[]>;
             };
           };
           try {
@@ -322,6 +325,21 @@ function vitePluginPhaseZApi(): Plugin {
               }
             }
           }
+          // IMP-08 B-3 — zoneSections override forward to CLI.
+          // Each entry becomes `--override-section-assignment ZONE=sid[,sid]`.
+          // Empty arrays and non-string sids are filtered out so the backend
+          // never receives bogus assignments from a partially-built UI state.
+          if (overrides?.zoneSections && typeof overrides.zoneSections === "object") {
+            for (const [zoneId, sids] of Object.entries(overrides.zoneSections)) {
+              if (!Array.isArray(sids)) continue;
+              const cleaned = sids.filter((s) => typeof s === "string" && s.trim());
+              if (cleaned.length === 0) continue;
+              cliArgs.push(
+                "--override-section-assignment",
+                `${zoneId}=${cleaned.join(",")}`
+              );
+            }
+          }
           console.log(
             `[phase-z-api] spawn pipeline: run_id=${runId}, mdx=${mdxPath}, args=${JSON.stringify(cliArgs.slice(2))}`
           );

src/phase_z2_composition.py

@@ -21,6 +21,7 @@ Pipeline 의 빠진 layer = MDX 덩어리들을 *최종 zone unit* 으로 묶는
 
 from __future__ import annotations
 
+import re
 from dataclasses import dataclass, field
 from pathlib import Path
 from typing import Optional
@@ -371,13 +372,20 @@ class CompositionUnit:
 # ─── Heading Tree ──────────────────────────────────────────────
 
 def derive_parent_id(section_id: str) -> Optional[str]:
-    """section_id 에서 parent 도출 — V4 키 컨벤션 기반.
+    """Section id -> parent id derivation by V4 key convention.
 
-    예시 (코멘트, 룰 X) :
-      - "04-2.1" → "04-2"  (decimal suffix → strip)
-      - "04-1"   → None    (top-level, no parent)
-      - "04"     → None
+    IMP-08 B-3 : canonical ordinal `${parent}-sub-${n}` recognised first;
+    legacy decimal `04-2.1` kept as fallback alias path.
+
+    Examples (illustrative, not rules) :
+      - "03-1-sub-2" -> "03-1"  (canonical ordinal, IMP-08)
+      - "04-2.1"     -> "04-2"  (decimal suffix, legacy V4 key style)
+      - "04-1"       -> None    (top-level, no parent)
+      - "04"         -> None
     """
+    m = re.fullmatch(r"(.+?)-sub-(\d+)", section_id)
+    if m:
+        return m.group(1)
     parts = section_id.split("-", 1)
     if len(parts) != 2:
         return None

tests/phase_z2/fixtures/build_layout_css/horizontal-2_default.yaml

@@ -0,0 +1,31 @@
+input:
+  layout_preset: horizontal-2
+  zones_data:
+  - position: top
+    template_id: MOCK_top
+    content_weight:
+      score: 0.5
+    min_height_px: 200
+  - position: bottom
+    template_id: MOCK_bottom
+    content_weight:
+      score: 0.5
+    min_height_px: 200
+  override_zone_geometries: null
+expected_layout_css:
+  areas: '"top" "bottom"'
+  cols: 1fr
+  rows: 286px 285px
+  heights_px:
+  - 286
+  - 285
+  widths_px:
+  - 1180
+  ratios:
+  - 0.489
+  - 0.487
+  width_ratios:
+  - 1.0
+  computation: min_height_first + content_weight_distribution
+  dynamic_rows: true
+  dynamic_cols: false

tests/phase_z2/fixtures/build_layout_css/horizontal-2_override.yaml

@@ -0,0 +1,41 @@
+input:
+  layout_preset: horizontal-2
+  zones_data:
+  - position: top
+    template_id: MOCK_top
+    content_weight:
+      score: 0.5
+    min_height_px: 200
+  - position: bottom
+    template_id: MOCK_bottom
+    content_weight:
+      score: 0.5
+    min_height_px: 200
+  override_zone_geometries:
+    top:
+      x: 0
+      y: 0
+      w: 1.0
+      h: 0.3
+    bottom:
+      x: 0
+      y: 0.3
+      w: 1.0
+      h: 0.7
+expected_layout_css:
+  areas: '"top" "bottom"'
+  cols: 1fr
+  rows: 176px 410px
+  heights_px:
+  - 176
+  - 410
+  widths_px:
+  - 1180
+  ratios:
+  - 0.3
+  - 0.7
+  width_ratios:
+  - 1.0
+  computation: user_override_geometry
+  dynamic_rows: true
+  dynamic_cols: false

tests/phase_z2/fixtures/build_layout_css/horizontal-2_weighted.yaml

@@ -0,0 +1,31 @@
+input:
+  layout_preset: horizontal-2
+  zones_data:
+  - position: top
+    template_id: MOCK_top
+    content_weight:
+      score: 0.8
+    min_height_px: 200
+  - position: bottom
+    template_id: MOCK_bottom
+    content_weight:
+      score: 0.2
+    min_height_px: 150
+  override_zone_geometries: null
+expected_layout_css:
+  areas: '"top" "bottom"'
+  cols: 1fr
+  rows: 377px 194px
+  heights_px:
+  - 377
+  - 194
+  widths_px:
+  - 1180
+  ratios:
+  - 0.644
+  - 0.332
+  width_ratios:
+  - 1.0
+  computation: min_height_first + content_weight_distribution
+  dynamic_rows: true
+  dynamic_cols: false

tests/phase_z2/fixtures/build_layout_css/vertical-2_default.yaml

@@ -0,0 +1,31 @@
+input:
+  layout_preset: vertical-2
+  zones_data:
+  - position: left
+    template_id: MOCK_left
+    content_weight:
+      score: 0.5
+    min_height_px: 200
+  - position: right
+    template_id: MOCK_right
+    content_weight:
+      score: 0.5
+    min_height_px: 200
+  override_zone_geometries: null
+expected_layout_css:
+  areas: '"left right"'
+  cols: 583px 583px
+  rows: 1fr
+  heights_px:
+  - 585
+  widths_px:
+  - 583
+  - 583
+  ratios:
+  - 1.0
+  width_ratios:
+  - 0.494
+  - 0.494
+  computation: content_weight_distribution_cols
+  dynamic_rows: false
+  dynamic_cols: true

tests/phase_z2/fixtures/build_layout_css/vertical-2_override.yaml

@@ -0,0 +1,41 @@
+input:
+  layout_preset: vertical-2
+  zones_data:
+  - position: left
+    template_id: MOCK_left
+    content_weight:
+      score: 0.5
+    min_height_px: 200
+  - position: right
+    template_id: MOCK_right
+    content_weight:
+      score: 0.5
+    min_height_px: 200
+  override_zone_geometries:
+    left:
+      x: 0
+      y: 0
+      w: 0.4
+      h: 1.0
+    right:
+      x: 0.4
+      y: 0
+      w: 0.6
+      h: 1.0
+expected_layout_css:
+  areas: '"left right"'
+  cols: 40.0fr 60.0fr
+  rows: 1fr
+  heights_px:
+  - 585
+  widths_px:
+  - 466
+  - 700
+  ratios:
+  - 1.0
+  width_ratios:
+  - 0.4
+  - 0.6
+  computation: user_override_geometry
+  dynamic_rows: false
+  dynamic_cols: true

tests/phase_z2/fixtures/build_layout_css/vertical-2_weighted.yaml

@@ -0,0 +1,31 @@
+input:
+  layout_preset: vertical-2
+  zones_data:
+  - position: left
+    template_id: MOCK_left
+    content_weight:
+      score: 0.7
+    min_height_px: 200
+  - position: right
+    template_id: MOCK_right
+    content_weight:
+      score: 0.3
+    min_height_px: 200
+  override_zone_geometries: null
+expected_layout_css:
+  areas: '"left right"'
+  cols: 816px 350px
+  rows: 1fr
+  heights_px:
+  - 585
+  widths_px:
+  - 816
+  - 350
+  ratios:
+  - 1.0
+  width_ratios:
+  - 0.692
+  - 0.297
+  computation: content_weight_distribution_cols
+  dynamic_rows: false
+  dynamic_cols: true

tests/phase_z2/fixtures/retry_gate/horizontal2_dynamic_rows.yaml

@@ -0,0 +1,24 @@
+case_id: horizontal2_dynamic_rows
+description: |
+  horizontal-2 layout with dynamic_rows=True must pass the IMP-09 retry
+  gate. The base trace should record retry_attempted=True (legacy
+  plan/rerender path continues). retry_skipped_reason MUST NOT contain
+  either of the IMP-09 gate skip strings.
+input_layout_css:
+  areas: '"top" "bottom"'
+  cols: 1fr
+  rows: 333px 238px
+  heights_px: [333, 238]
+  widths_px: [1180]
+  ratios: [0.569, 0.407]
+  width_ratios: [1.0]
+  dynamic_rows: true
+  dynamic_cols: false
+router_decision:
+  router_active: true
+  proposed_actions_summary: [zone_ratio_retry]
+expected_gate:
+  retry_attempted: true
+  retry_skipped_reason_excludes:
+    - "dynamic_cols"
+    - "fr_default_from_preset"

tests/phase_z2/fixtures/retry_gate/single_fr_default.yaml

@@ -0,0 +1,26 @@
+case_id: single_fr_default
+description: |
+  Post-IMP-09 PR 2 the only preset that still falls through to
+  fr_default_from_preset is `single` (top-1-bottom-2 / top-2-bottom-1 /
+  left-1-right-2 / left-2-right-1 / grid-2x2 were promoted to 2-D
+  dynamic). This fixture exercises the IMP-09 retry-gate fr_default
+  skip path using a layout_css with dynamic_rows=False AND
+  dynamic_cols=False (the surviving fr_default signature). Row-axis
+  retry must be skipped with a fr_default_from_preset skip reason.
+input_layout_css:
+  areas: '"top top" "bottom-left bottom-right"'
+  cols: 1fr 1fr
+  rows: 1fr 1fr
+  heights_px: [285, 286]
+  widths_px: [583, 583]
+  ratios: [0.487, 0.489]
+  width_ratios: [0.494, 0.494]
+  dynamic_rows: false
+  dynamic_cols: false
+router_decision:
+  router_active: true
+  proposed_actions_summary: [zone_ratio_retry]
+expected_gate:
+  retry_attempted: false
+  retry_skipped_reason_contains:
+    - "fr_default_from_preset"

tests/phase_z2/fixtures/retry_gate/vertical2_dynamic_cols.yaml

@@ -0,0 +1,24 @@
+case_id: vertical2_dynamic_cols
+description: |
+  vertical-2 layout with dynamic_cols=True must be skipped by the
+  IMP-09 retry gate before plan/rerender, because the existing
+  apply_retry_to_layout_css mutates only row-axis fields and would
+  produce a misleading trace if it ran on a column-dynamic layout.
+input_layout_css:
+  areas: '"left right"'
+  cols: 583px 583px
+  rows: 1fr
+  heights_px: [585]
+  widths_px: [583, 583]
+  ratios: [1.0]
+  width_ratios: [0.494, 0.494]
+  dynamic_rows: false
+  dynamic_cols: true
+router_decision:
+  router_active: true
+  proposed_actions_summary: [zone_ratio_retry]
+expected_gate:
+  retry_attempted: false
+  retry_skipped_reason_contains:
+    - "dynamic_cols"
+    - "IMP-09"

tests/phase_z2/test_build_layout_css_pr1.py

@@ -0,0 +1,246 @@
+"""IMP-09 PR 1 — build_layout_css contract tests.
+
+Verifies horizontal-2 byte-identity for the legacy grid strings
+(areas / cols / rows) and that every return path now carries the new
+length-locked col-axis keys (widths_px / width_ratios / dynamic_cols).
+"""
+from __future__ import annotations
+
+import pytest
+
+from src.phase_z2_pipeline import (
+    GRID_GAP,
+    SLIDE_BODY_HEIGHT,
+    SLIDE_BODY_WIDTH,
+    build_layout_css,
+)
+
+
+def _zone(position: str, score: float, min_h: int = 100) -> dict:
+    return {
+        "position": position,
+        "template_id": f"MOCK_{position}",
+        "content_weight": {"score": score},
+        "min_height_px": min_h,
+    }
+
+
+# ────────────────────── new-key contract ──────────────────────
+
+
+NEW_KEYS = {"widths_px", "width_ratios", "dynamic_cols"}
+
+
+def test_all_presets_carry_new_col_axis_keys():
+    """Every PR 1 return path must include widths_px / width_ratios /
+    dynamic_cols, and heights_px / widths_px must be length-locked to
+    the catalog grid (R rows, C cols)."""
+    cases = [
+        ("single", [_zone("primary", 1.0)]),
+        ("horizontal-2", [_zone("top", 0.6), _zone("bottom", 0.4)]),
+        ("vertical-2", [_zone("left", 0.5), _zone("right", 0.5)]),
+        ("top-1-bottom-2", [
+            _zone("top", 0.5),
+            _zone("bottom-left", 0.25),
+            _zone("bottom-right", 0.25),
+        ]),
+        ("grid-2x2", [
+            _zone("top-left", 0.25),
+            _zone("top-right", 0.25),
+            _zone("bottom-left", 0.25),
+            _zone("bottom-right", 0.25),
+        ]),
+    ]
+    for preset, zones in cases:
+        result = build_layout_css(preset, zones)
+        missing = NEW_KEYS - set(result)
+        assert not missing, f"{preset} missing new keys: {missing}"
+        # heights_px / widths_px never empty in PR 1 (length-locked).
+        assert len(result["heights_px"]) > 0, f"{preset} empty heights_px"
+        assert len(result["widths_px"]) > 0, f"{preset} empty widths_px"
+
+
+# ────────────────────── horizontal-2 byte-identity ──────────────────────
+
+
+def test_horizontal_2_grid_strings_match_legacy():
+    zones = [_zone("top", 0.6), _zone("bottom", 0.4)]
+    result = build_layout_css("horizontal-2", zones)
+
+    # Legacy contract: areas / cols / rows strings preserved.
+    assert result["areas"] == '"top" "bottom"'
+    assert result["cols"] == "1fr"
+    assert result["rows"].count("px") == 2
+
+    # heights_px sum to body height; ratios consistent.
+    assert sum(result["heights_px"]) == SLIDE_BODY_HEIGHT - GRID_GAP
+    assert result["dynamic_rows"] is True
+    assert result["dynamic_cols"] is False
+
+    # New col-axis defaults: full body width, ratio 1.0.
+    assert result["widths_px"] == [SLIDE_BODY_WIDTH]
+    assert result["width_ratios"] == [1.0]
+
+
+def test_horizontal_2_override_preserves_rows():
+    zones = [_zone("top", 0.6), _zone("bottom", 0.4)]
+    override = {
+        "top": {"x": 0, "y": 0, "w": 1.0, "h": 0.3},
+        "bottom": {"x": 0, "y": 0.3, "w": 1.0, "h": 0.7},
+    }
+    result = build_layout_css(
+        "horizontal-2", zones, override_zone_geometries=override
+    )
+    assert result["computation"] == "user_override_geometry"
+    assert result["dynamic_rows"] is True
+    assert result["dynamic_cols"] is False
+    assert result["heights_px"][0] < result["heights_px"][1]
+    assert result["widths_px"] == [SLIDE_BODY_WIDTH]
+    # Override ratio target.
+    assert result["ratios"] == [0.3, 0.7]
+
+
+# ────────────────────── vertical-2 new dynamic ──────────────────────
+
+
+def test_vertical_2_normal_produces_dynamic_cols():
+    zones = [_zone("left", 0.7), _zone("right", 0.3)]
+    result = build_layout_css("vertical-2", zones)
+    assert result["dynamic_rows"] is False
+    assert result["dynamic_cols"] is True
+    # cols string is px-based (no fr).
+    assert "fr" not in result["cols"]
+    assert result["cols"].count("px") == 2
+    # Heights span full body in a single row.
+    assert result["heights_px"] == [SLIDE_BODY_HEIGHT]
+    # Widths reflect 70/30 weight split.
+    assert result["widths_px"][0] > result["widths_px"][1]
+
+
+def test_vertical_2_override_keeps_fr_cols_legacy():
+    """PR 1 v-2 override path keeps legacy fr-string cols but now
+    populates widths_px in pixels for downstream consumers."""
+    zones = [_zone("left", 0.5), _zone("right", 0.5)]
+    override = {
+        "left": {"x": 0, "y": 0, "w": 0.4, "h": 1.0},
+        "right": {"x": 0.4, "y": 0, "w": 0.6, "h": 1.0},
+    }
+    result = build_layout_css(
+        "vertical-2", zones, override_zone_geometries=override
+    )
+    assert result["computation"] == "user_override_geometry"
+    assert "fr" in result["cols"]
+    assert result["dynamic_cols"] is True
+    assert result["dynamic_rows"] is False
+    # widths_px now populated.
+    assert len(result["widths_px"]) == 2
+    assert sum(result["widths_px"]) == SLIDE_BODY_WIDTH - GRID_GAP
+    assert result["width_ratios"] == [0.4, 0.6]
+
+
+# ───────────────── PR 2: 5 in-scope 2-D presets dynamic ─────────────────
+
+
+_TWO_D_PRESETS = [
+    "top-1-bottom-2",
+    "top-2-bottom-1",
+    "left-1-right-2",
+    "left-2-right-1",
+    "grid-2x2",
+]
+
+
+def _zones_for(preset: str) -> list[dict]:
+    """Default zone fixtures (positions per LAYOUT_PRESETS, equal score)."""
+    if preset == "top-1-bottom-2":
+        return [_zone("top", 0.5), _zone("bottom-left", 0.25),
+                _zone("bottom-right", 0.25)]
+    if preset == "top-2-bottom-1":
+        return [_zone("top-left", 0.3), _zone("top-right", 0.2),
+                _zone("bottom", 0.5)]
+    if preset == "left-1-right-2":
+        return [_zone("left", 0.5), _zone("right-top", 0.3),
+                _zone("right-bottom", 0.2)]
+    if preset == "left-2-right-1":
+        return [_zone("left-top", 0.3), _zone("left-bottom", 0.2),
+                _zone("right", 0.5)]
+    if preset == "grid-2x2":
+        return [_zone("top-left", 0.25), _zone("top-right", 0.25),
+                _zone("bottom-left", 0.25), _zone("bottom-right", 0.25)]
+    raise ValueError(f"no _zones_for({preset!r})")
+
+
+@pytest.mark.parametrize("preset", _TWO_D_PRESETS)
+def test_two_d_preset_promoted_to_dynamic(preset):
+    """PR 2 — T / inverted-T / side-T-{left,right} / 2x2 must dispatch
+    to _build_2d_dynamic (computation=='2d_dynamic_aggregated') with
+    dynamic_rows=True AND dynamic_cols=True, and grid-template strings
+    in pixels."""
+    result = build_layout_css(preset, _zones_for(preset))
+    assert result["computation"] == "2d_dynamic_aggregated"
+    assert result["dynamic_rows"] is True
+    assert result["dynamic_cols"] is True
+    # Both axes pixel-based.
+    assert "fr" not in result["rows"]
+    assert "fr" not in result["cols"]
+    assert result["rows"].count("px") == 2
+    assert result["cols"].count("px") == 2
+    # Length contract — R=2 rows, C=2 cols for all 5 in-scope presets.
+    assert len(result["heights_px"]) == 2
+    assert len(result["widths_px"]) == 2
+
+
+@pytest.mark.parametrize("preset", _TWO_D_PRESETS)
+def test_two_d_preset_override_dispatches_via_helper(preset):
+    """Override on a 2-D preset must route through
+    _override_to_grid_tracks (computation=='user_override_geometry')
+    with both axes marked dynamic."""
+    zones = _zones_for(preset)
+    positions = [z["position"] for z in zones]
+    override = {pos: {"x": 0, "y": 0, "w": 0.5, "h": 0.5} for pos in positions}
+    result = build_layout_css(preset, zones, override_zone_geometries=override)
+    assert result["computation"] == "user_override_geometry"
+    assert result["dynamic_rows"] is True
+    assert result["dynamic_cols"] is True
+    assert len(result["heights_px"]) == 2
+    assert len(result["widths_px"]) == 2
+    # Total of axis cell sums equals body minus inter-track gap.
+    assert sum(result["heights_px"]) == SLIDE_BODY_HEIGHT - GRID_GAP
+    assert sum(result["widths_px"]) == SLIDE_BODY_WIDTH - GRID_GAP
+
+
+@pytest.mark.parametrize("preset", _TWO_D_PRESETS)
+def test_two_d_preset_override_invalid_falls_back_to_dynamic(preset):
+    """Zero-only override on a 2-D preset must fall back to dynamic
+    normal-path solvers on both axes; computation still labeled
+    user_override_geometry (override invocation context preserved)
+    and raw_zone_layout.{row_source,col_source}=='dynamic_fallback'."""
+    zones = _zones_for(preset)
+    positions = [z["position"] for z in zones]
+    # Zero-only override — must trigger per-axis fallback to solvers.
+    override = {pos: {"x": 0, "y": 0, "w": 0.0, "h": 0.0} for pos in positions}
+    result = build_layout_css(preset, zones, override_zone_geometries=override)
+    assert result["computation"] == "user_override_geometry"
+    rzl = result["raw_zone_layout"]
+    assert rzl["row_source"] == "dynamic_fallback"
+    assert rzl["col_source"] == "dynamic_fallback"
+    # Result must match what dynamic normal path would produce.
+    normal = build_layout_css(preset, zones)
+    assert result["heights_px"] == normal["heights_px"]
+    assert result["widths_px"] == normal["widths_px"]
+
+
+# ────────────────────── fr_default sink (PR 2 — single only) ──────────────────────
+
+
+def test_single_remains_fr_default_sink():
+    """After PR 2 the only preset that still falls through to
+    _build_fr_default is `single` (PR 3 will promote it). Length
+    contract (R=1, C=1) must still hold."""
+    zones = [_zone("primary", 1.0)]
+    result = build_layout_css("single", zones)
+    assert result["computation"] == "fr_default_from_preset"
+    assert result["dynamic_rows"] is False
+    assert result["dynamic_cols"] is False
+    assert len(result["heights_px"]) == 1
+    assert len(result["widths_px"]) == 1

tests/phase_z2/test_compute_per_zone_geometry.py

@@ -0,0 +1,101 @@
+"""IMP-09 PR 1 — _compute_per_zone_geometry tests (1-D paths).
+
+Verifies the unified per-zone geometry aggregator on horizontal-2 and
+vertical-2 (the two 1-D presets active in PR 1). 2-D spanning zone
+cases (T / 2x2) are exercised in PR 2.
+
+The helper aggregates grid-track sizes into per-zone dimensions and
+must produce length-locked outputs:
+  - layout_css["heights_px"] length == R (parsed css_areas rows)
+  - layout_css["widths_px"]  length == C (parsed css_areas cols)
+"""
+from __future__ import annotations
+
+import pytest
+
+from src.phase_z2_pipeline import (
+    GRID_GAP,
+    SLIDE_BODY_HEIGHT,
+    SLIDE_BODY_WIDTH,
+    _compute_per_zone_geometry,
+    build_layout_css,
+)
+
+
+def _zone(position: str, score: float) -> dict:
+    return {
+        "position": position,
+        "template_id": f"MOCK_{position}",
+        "content_weight": {"score": score},
+        "min_height_px": 100,
+    }
+
+
+def test_horizontal_2_per_zone_widths_match_slide_body():
+    zones = [_zone("top", 0.6), _zone("bottom", 0.4)]
+    layout_css = build_layout_css("horizontal-2", zones)
+    debug_zones = [{"position": "top"}, {"position": "bottom"}]
+    per_zone = _compute_per_zone_geometry(layout_css, debug_zones, GRID_GAP)
+
+    # Both zones share the single column => width == SLIDE_BODY_WIDTH.
+    assert per_zone[0]["zone_width_px"] == SLIDE_BODY_WIDTH
+    assert per_zone[1]["zone_width_px"] == SLIDE_BODY_WIDTH
+    # Heights mirror layout_css.heights_px.
+    assert per_zone[0]["zone_height_px"] == layout_css["heights_px"][0]
+    assert per_zone[1]["zone_height_px"] == layout_css["heights_px"][1]
+
+
+def test_vertical_2_per_zone_heights_match_slide_body():
+    zones = [_zone("left", 0.5), _zone("right", 0.5)]
+    layout_css = build_layout_css("vertical-2", zones)
+    debug_zones = [{"position": "left"}, {"position": "right"}]
+    per_zone = _compute_per_zone_geometry(layout_css, debug_zones, GRID_GAP)
+
+    # Both zones share the single row => height == SLIDE_BODY_HEIGHT.
+    assert per_zone[0]["zone_height_px"] == SLIDE_BODY_HEIGHT
+    assert per_zone[1]["zone_height_px"] == SLIDE_BODY_HEIGHT
+    # Widths mirror layout_css.widths_px.
+    assert per_zone[0]["zone_width_px"] == layout_css["widths_px"][0]
+    assert per_zone[1]["zone_width_px"] == layout_css["widths_px"][1]
+
+
+def test_heights_px_length_mismatch_raises():
+    layout_css = {
+        "areas": '"top" "bottom"',
+        "heights_px": [300],          # wrong length, expected 2
+        "widths_px": [SLIDE_BODY_WIDTH],
+    }
+    with pytest.raises(ValueError, match="heights_px length"):
+        _compute_per_zone_geometry(
+            layout_css, [{"position": "top"}], GRID_GAP
+        )
+
+
+def test_widths_px_length_mismatch_raises():
+    layout_css = {
+        "areas": '"left right"',
+        "heights_px": [SLIDE_BODY_HEIGHT],
+        "widths_px": [600],            # wrong length, expected 2
+    }
+    with pytest.raises(ValueError, match="widths_px length"):
+        _compute_per_zone_geometry(
+            layout_css, [{"position": "left"}], GRID_GAP
+        )
+
+
+def test_unknown_position_raises():
+    zones = [_zone("top", 0.5), _zone("bottom", 0.5)]
+    layout_css = build_layout_css("horizontal-2", zones)
+    debug_zones = [{"position": "ghost"}]
+    with pytest.raises(ValueError, match="not present in css_areas"):
+        _compute_per_zone_geometry(layout_css, debug_zones, GRID_GAP)
+
+
+def test_fr_default_single_returns_full_body():
+    # 'single' is the fr_default sink in PR 1; widths_px / heights_px
+    # must still be populated (length 1 each).
+    layout_css = build_layout_css("single", [_zone("primary", 1.0)])
+    debug_zones = [{"position": "primary"}]
+    per_zone = _compute_per_zone_geometry(layout_css, debug_zones, GRID_GAP)
+    assert per_zone[0]["zone_height_px"] == SLIDE_BODY_HEIGHT
+    assert per_zone[0]["zone_width_px"] == SLIDE_BODY_WIDTH

tests/phase_z2/test_compute_zone_layout_cols.py

@@ -0,0 +1,76 @@
+"""IMP-09 PR 1 — compute_zone_layout_cols tests.
+
+Column-axis weight-only solver. Mirrors compute_zone_layout for rows.
+No min_width_px contract exists in frame_contracts.yaml (verified
+during Stage 2), so column distribution is purely content_weight.
+"""
+from __future__ import annotations
+
+from src.phase_z2_pipeline import (
+    GRID_GAP,
+    SLIDE_BODY_WIDTH,
+    compute_zone_layout_cols,
+)
+
+
+def _zone(position: str, score: float) -> dict:
+    return {
+        "position": position,
+        "template_id": f"MOCK_{position}",
+        "content_weight": {"score": score},
+    }
+
+
+def test_empty_zones_returns_empty_result():
+    result = compute_zone_layout_cols([])
+    assert result["widths_px"] == []
+    assert result["width_ratios"] == []
+
+
+def test_two_equal_zones_split_evenly():
+    zones = [_zone("left", 0.5), _zone("right", 0.5)]
+    result = compute_zone_layout_cols(zones)
+    available = SLIDE_BODY_WIDTH - GRID_GAP  # one gap between two zones
+    assert sum(result["widths_px"]) == available
+    assert result["widths_px"][0] == result["widths_px"][1]
+    assert result["computation"] == "content_weight_distribution_cols"
+
+
+def test_asymmetric_weights_distribute_by_ratio():
+    zones = [_zone("left", 0.8), _zone("right", 0.2)]
+    result = compute_zone_layout_cols(zones)
+    available = SLIDE_BODY_WIDTH - GRID_GAP
+    assert sum(result["widths_px"]) == available
+    # left should be ~4x right
+    assert result["widths_px"][0] > result["widths_px"][1] * 3
+
+
+def test_zero_weight_guard_equal_split():
+    zones = [_zone("left", 0.0), _zone("right", 0.0)]
+    result = compute_zone_layout_cols(zones)
+    available = SLIDE_BODY_WIDTH - GRID_GAP
+    assert sum(result["widths_px"]) == available
+    assert result["widths_px"][0] == result["widths_px"][1]
+    # weight_shares fallback to equal share.
+    assert result["weight_shares"] == [0.5, 0.5]
+
+
+def test_integer_rounding_absorbed_by_last_zone():
+    # Three zones with weights that don't divide evenly.
+    zones = [
+        _zone("a", 0.333333),
+        _zone("b", 0.333333),
+        _zone("c", 0.333334),
+    ]
+    result = compute_zone_layout_cols(zones)
+    available = SLIDE_BODY_WIDTH - 2 * GRID_GAP
+    assert sum(result["widths_px"]) == available
+
+
+def test_width_ratios_match_total_width():
+    zones = [_zone("left", 0.6), _zone("right", 0.4)]
+    result = compute_zone_layout_cols(zones)
+    # width_ratios should be widths_px / SLIDE_BODY_WIDTH (not / available)
+    assert abs(
+        result["width_ratios"][0] - result["widths_px"][0] / SLIDE_BODY_WIDTH
+    ) < 1e-3

tests/phase_z2/test_fixtures_loader.py

@@ -0,0 +1,100 @@
+"""IMP-09 PR 1 — fixture-driven regression checks.
+
+Loads the YAML snapshots under tests/phase_z2/fixtures/ and exercises
+build_layout_css + _attempt_zone_ratio_retry against them. Any drift
+in IMP-09 output forces a fixture refresh, which is the lock surface
+called out in Stage 3 round 4 §5.
+"""
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+import yaml
+
+from src.phase_z2_pipeline import _attempt_zone_ratio_retry, build_layout_css
+
+
+FIXTURES_DIR = Path(__file__).parent / "fixtures"
+
+
+def _load_yaml(path: Path) -> dict:
+    with path.open(encoding="utf-8") as f:
+        return yaml.safe_load(f)
+
+
+# ──────────────────────── build_layout_css fixtures ────────────────────────
+
+
+_BUILD_DIR = FIXTURES_DIR / "build_layout_css"
+_BUILD_FIXTURES = sorted(_BUILD_DIR.glob("*.yaml")) if _BUILD_DIR.exists() else []
+
+
+@pytest.mark.parametrize(
+    "fixture_path",
+    _BUILD_FIXTURES,
+    ids=[p.stem for p in _BUILD_FIXTURES],
+)
+def test_build_layout_css_matches_fixture(fixture_path: Path):
+    payload = _load_yaml(fixture_path)
+    inp = payload["input"]
+    expected = payload["expected_layout_css"]
+
+    result = build_layout_css(
+        inp["layout_preset"],
+        inp["zones_data"],
+        override_zone_geometries=inp.get("override_zone_geometries"),
+    )
+    # raw_zone_layout is intentionally not snapshotted (contains
+    # solver internals); compare the rest.
+    actual = {k: v for k, v in result.items() if k != "raw_zone_layout"}
+    assert actual == expected, (
+        f"layout_css drift in fixture {fixture_path.name}:\n"
+        f"  expected={expected}\n  actual={actual}"
+    )
+
+
+# ────────────────────────── retry_gate fixtures ──────────────────────────
+
+
+_RETRY_DIR = FIXTURES_DIR / "retry_gate"
+_RETRY_FIXTURES = sorted(_RETRY_DIR.glob("*.yaml")) if _RETRY_DIR.exists() else []
+
+
+@pytest.mark.parametrize(
+    "fixture_path",
+    _RETRY_FIXTURES,
+    ids=[p.stem for p in _RETRY_FIXTURES],
+)
+def test_retry_gate_matches_fixture(fixture_path: Path, tmp_path: Path):
+    payload = _load_yaml(fixture_path)
+    layout_css = payload["input_layout_css"]
+    router_decision = payload["router_decision"]
+    expected = payload["expected_gate"]
+
+    trace = _attempt_zone_ratio_retry(
+        run_dir=tmp_path,
+        out_path=tmp_path / "final.html",
+        slide_title="fixture",
+        slide_footer=None,
+        zones_data=[],
+        debug_zones=[],
+        layout_preset="fixture",
+        layout_css=layout_css,
+        overflow={},
+        fit_classification={},
+        router_decision=router_decision,
+        gap_px=14,
+    )
+
+    assert trace["retry_attempted"] == expected["retry_attempted"]
+    skip_reason = trace.get("retry_skipped_reason")
+    for needle in expected.get("retry_skipped_reason_contains", []):
+        assert skip_reason is not None and needle in skip_reason, (
+            f"expected {needle!r} in retry_skipped_reason, got {skip_reason!r}"
+        )
+    for forbidden in expected.get("retry_skipped_reason_excludes", []):
+        if skip_reason is not None:
+            assert forbidden not in skip_reason, (
+                f"forbidden {forbidden!r} found in retry_skipped_reason {skip_reason!r}"
+            )

tests/phase_z2/test_parse_css_areas_validation.py

@@ -0,0 +1,71 @@
+"""IMP-09 PR 1 — _parse_css_areas strict validation tests.
+
+Covers the four ValueError cases declared in the Stage 3 round 4 lock
+(plan §2-D): empty input, no quoted rows, empty row tokens, and
+non-rectangular grids. Also exercises positive parsing on all 8
+catalog presets so any future catalog drift in row/col counts is
+caught here.
+"""
+from __future__ import annotations
+
+import pytest
+
+from src.phase_z2_pipeline import _parse_css_areas
+
+
+def test_parse_empty_string_raises():
+    with pytest.raises(ValueError, match="no quoted row strings"):
+        _parse_css_areas("")
+
+
+def test_parse_no_quotes_raises():
+    with pytest.raises(ValueError, match="no quoted row strings"):
+        _parse_css_areas("top top bottom-left bottom-right")
+
+
+def test_parse_empty_row_raises():
+    # Whitespace-only quoted row -> tokens list is empty.
+    with pytest.raises(ValueError, match="empty row"):
+        _parse_css_areas('"   "')
+
+
+def test_parse_non_rectangular_raises():
+    # First row has 1 token, second row has 2 tokens.
+    with pytest.raises(ValueError, match="non-rectangular"):
+        _parse_css_areas('"top" "bottom-left bottom-right"')
+
+
+def test_parse_single_zone():
+    rows, seen = _parse_css_areas('"primary"')
+    assert rows == [["primary"]]
+    assert seen == ["primary"]
+
+
+def test_parse_horizontal_2():
+    rows, seen = _parse_css_areas('"top" "bottom"')
+    assert rows == [["top"], ["bottom"]]
+    assert seen == ["top", "bottom"]
+
+
+def test_parse_vertical_2():
+    rows, seen = _parse_css_areas('"left right"')
+    assert rows == [["left", "right"]]
+    assert seen == ["left", "right"]
+
+
+def test_parse_top_1_bottom_2_span():
+    rows, seen = _parse_css_areas('"top top" "bottom-left bottom-right"')
+    assert rows == [["top", "top"], ["bottom-left", "bottom-right"]]
+    # 'top' should appear once in seen even though it occupies two cells.
+    assert seen == ["top", "bottom-left", "bottom-right"]
+
+
+def test_parse_grid_2x2_four_zones():
+    rows, seen = _parse_css_areas(
+        '"top-left top-right" "bottom-left bottom-right"'
+    )
+    assert rows == [
+        ["top-left", "top-right"],
+        ["bottom-left", "bottom-right"],
+    ]
+    assert seen == ["top-left", "top-right", "bottom-left", "bottom-right"]

tests/phase_z2/test_parse_fr_string.py

@@ -0,0 +1,49 @@
+"""IMP-09 PR 1 — _parse_fr_string tests.
+
+Catalog presets only use `1fr` / `1fr 1fr` specs (verified
+templates/phase_z2/layouts/layouts.yaml). The helper must reject
+non-fr tokens and round to integer pixel sizes summing to `total`.
+"""
+from __future__ import annotations
+
+import pytest
+
+from src.phase_z2_pipeline import _parse_fr_string
+
+
+def test_single_fr_returns_full_total():
+    assert _parse_fr_string("1fr", 585) == [585]
+
+
+def test_two_equal_fr_splits_evenly():
+    result = _parse_fr_string("1fr 1fr", 1180)
+    assert result == [590, 590]
+    assert sum(result) == 1180
+
+
+def test_unequal_fr_distributes_by_ratio():
+    result = _parse_fr_string("2fr 1fr", 300)
+    assert sum(result) == 300
+    assert result[0] > result[1]
+
+
+def test_rounding_absorbed_by_last_track():
+    # 1fr 1fr 1fr / total=100 -> 33,33,33 + diff 1 absorbed by last.
+    result = _parse_fr_string("1fr 1fr 1fr", 100)
+    assert sum(result) == 100
+    assert result == [33, 33, 34]
+
+
+def test_non_fr_token_raises():
+    with pytest.raises(ValueError, match="non-fr token"):
+        _parse_fr_string("200px 1fr", 1000)
+
+
+def test_empty_spec_raises():
+    with pytest.raises(ValueError, match="empty spec"):
+        _parse_fr_string("", 1000)
+
+
+def test_zero_fr_raises():
+    with pytest.raises(ValueError, match="total fr"):
+        _parse_fr_string("0fr 0fr", 1000)

tests/phase_z2/test_retry_gate.py

@@ -0,0 +1,129 @@
+"""IMP-09 PR 1 — retry gate tests (_attempt_zone_ratio_retry early exit).
+
+Stage 3 round 4 lock §2-A: row-axis retry must skip when layout has
+dynamic_cols=True (2-D topology) OR dynamic_rows=False (fr_default
+sink). The horizontal-2 path (dynamic_rows=True, dynamic_cols=False)
+must still proceed through the gate.
+
+These tests exercise the gate by routing the request through
+_attempt_zone_ratio_retry with router_active=True + proposed
+zone_ratio_retry — but with layout_css fields that should trip the
+gate. We confirm the early skip by asserting retry_attempted==False
+and retry_skipped_reason content.
+"""
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+from src.phase_z2_pipeline import _attempt_zone_ratio_retry
+
+
+_ROUTER_ACTIVE = {
+    "router_active": True,
+    "proposed_actions_summary": ["zone_ratio_retry"],
+}
+
+
+def _dummy_kwargs(layout_css: dict, tmp_path: Path) -> dict:
+    """All params required by _attempt_zone_ratio_retry. Only
+    `layout_css` and `router_decision` matter pre-gate."""
+    return {
+        "run_dir": tmp_path,
+        "out_path": tmp_path / "final.html",
+        "slide_title": "test",
+        "slide_footer": None,
+        "zones_data": [],
+        "debug_zones": [],
+        "layout_preset": "horizontal-2",
+        "layout_css": layout_css,
+        "overflow": {},
+        "fit_classification": {},
+        "router_decision": _ROUTER_ACTIVE,
+        "gap_px": 14,
+    }
+
+
+def test_vertical_2_dynamic_cols_skips_retry(tmp_path):
+    layout_css = {
+        "areas": '"left right"',
+        "cols": "583px 583px",
+        "rows": "1fr",
+        "heights_px": [585],
+        "widths_px": [583, 583],
+        "ratios": [1.0],
+        "width_ratios": [0.494, 0.494],
+        "dynamic_rows": False,
+        "dynamic_cols": True,
+    }
+    trace = _attempt_zone_ratio_retry(**_dummy_kwargs(layout_css, tmp_path))
+    assert trace["retry_attempted"] is False
+    assert "dynamic_cols" in trace["retry_skipped_reason"]
+    assert "IMP-09" in trace["retry_skipped_reason"]
+
+
+def test_fr_default_sink_skips_retry(tmp_path):
+    # PR 1 single / T-shape / 2x2 fall through to fr_default and must
+    # not enter row-only retry plan.
+    layout_css = {
+        "areas": '"top top" "bottom-left bottom-right"',
+        "cols": "1fr 1fr",
+        "rows": "1fr 1fr",
+        "heights_px": [285, 286],
+        "widths_px": [583, 583],
+        "ratios": [0.487, 0.489],
+        "width_ratios": [0.494, 0.494],
+        "dynamic_rows": False,
+        "dynamic_cols": False,
+    }
+    trace = _attempt_zone_ratio_retry(**_dummy_kwargs(layout_css, tmp_path))
+    assert trace["retry_attempted"] is False
+    assert "fr_default_from_preset" in trace["retry_skipped_reason"]
+
+
+def test_horizontal_2_dynamic_rows_passes_gate(tmp_path):
+    """horizontal-2 with dynamic_rows=True must pass the gate. The
+    test does not need plan_zone_ratio_retry to succeed; it only
+    asserts the gate did not early-skip with one of the new
+    skip reasons."""
+    layout_css = {
+        "areas": '"top" "bottom"',
+        "cols": "1fr",
+        "rows": "333px 238px",
+        "heights_px": [333, 238],
+        "widths_px": [1180],
+        "ratios": [0.569, 0.407],
+        "width_ratios": [1.0],
+        "dynamic_rows": True,
+        "dynamic_cols": False,
+    }
+    # plan_zone_ratio_retry will return None because debug_zones is
+    # empty, so retry_attempted=True but plan==None.
+    trace = _attempt_zone_ratio_retry(**_dummy_kwargs(layout_css, tmp_path))
+    assert trace["retry_attempted"] is True
+    # The gate was passed; skip reason (if any) is the legacy
+    # plan-failure reason, not the new gate reasons.
+    skip_reason = trace.get("retry_skipped_reason")
+    if skip_reason is not None:
+        assert "dynamic_cols" not in skip_reason
+        assert "fr_default_from_preset" not in skip_reason
+
+
+def test_router_inactive_skips_before_gate(tmp_path):
+    """When router_active=False, the early skip happens before the
+    new IMP-09 gate. Verify the existing behavior is unchanged."""
+    layout_css = {
+        "areas": '"left right"',
+        "dynamic_rows": False,
+        "dynamic_cols": True,
+        "heights_px": [585],
+        "widths_px": [583, 583],
+        "ratios": [1.0],
+        "width_ratios": [0.5, 0.5],
+    }
+    kwargs = _dummy_kwargs(layout_css, tmp_path)
+    kwargs["router_decision"] = {"router_active": False}
+    trace = _attempt_zone_ratio_retry(**kwargs)
+    assert trace["retry_attempted"] is False
+    assert "router_active=False" in trace["retry_skipped_reason"]

tests/test_phase_z2_subsection_schema.py

@@ -0,0 +1,239 @@
+"""IMP-08 B-3 sub-section drag/drop — schema + V4 alias resolver tests.
+
+Fully synthetic per Codex #7 generalization guardrail:
+NO real catalog template_id / frame_id, NO ``v4_full32_result.yaml`` dependency,
+NO MDX-specific section ids beyond canonical id format.
+
+Locked scope (Stage 3 R8) :
+  A. ``derive_parent_id`` canonical ordinal recognition + legacy decimal fallback.
+  B. ``_resolve_v4_section_key`` exact > alias > None (no parent/sibling promotion).
+  C. ``align_sections_to_v4_granularity`` canonical ordinal id emit + N-R5
+     decimal-only alias guard + MdxSection default-construction stability.
+"""
+from __future__ import annotations
+
+from src.phase_z2_composition import derive_parent_id
+from src.phase_z2_pipeline import (
+    MdxSection,
+    _resolve_v4_section_key,
+    align_sections_to_v4_granularity,
+)
+
+
+# ─── A. derive_parent_id ────────────────────────────────────────────────────
+
+
+def test_derive_parent_id_ordinal_sub():
+    assert derive_parent_id("03-1-sub-2") == "03-1"
+    assert derive_parent_id("04-2-sub-1") == "04-2"
+
+
+def test_derive_parent_id_decimal_legacy_alias():
+    # Legacy V4 decimal id retains existing behaviour for alias path.
+    assert derive_parent_id("04-2.1") == "04-2"
+
+
+def test_derive_parent_id_top_level_none():
+    assert derive_parent_id("04-1") is None
+    assert derive_parent_id("04") is None
+    assert derive_parent_id("nonsense") is None
+
+
+# ─── B. _resolve_v4_section_key ─────────────────────────────────────────────
+
+
+def _fake_v4(*keys):
+    return {"mdx_sections": {k: {"judgments_full32": []} for k in keys}}
+
+
+def test_alias_resolver_exact_match_wins():
+    v4 = _fake_v4("04-2-sub-1", "04-2.1")
+    assert _resolve_v4_section_key(v4, "04-2-sub-1") == "04-2-sub-1"
+    assert (
+        _resolve_v4_section_key(v4, "04-2-sub-1", alias_keys=["04-2.1"])
+        == "04-2-sub-1"
+    )
+
+
+def test_alias_resolver_decimal_alias_when_metadata_present():
+    v4 = _fake_v4("04-2.1")
+    assert (
+        _resolve_v4_section_key(v4, "04-2-sub-1", alias_keys=["04-2.1"])
+        == "04-2.1"
+    )
+
+
+def test_alias_resolver_no_parent_promotion():
+    # parent V4 entry must not be promoted into a sibling sub-section lookup.
+    v4 = _fake_v4("04-2")
+    assert _resolve_v4_section_key(v4, "04-2-sub-1") is None
+    assert (
+        _resolve_v4_section_key(v4, "04-2-sub-1", alias_keys=["04-2"])
+        == "04-2"
+    )  # alias is opt-in; only resolves when caller explicitly provides it
+
+
+def test_alias_resolver_no_sibling_promotion():
+    # sibling sub-section entry must not be auto-promoted without an alias.
+    v4 = _fake_v4("04-2-sub-2")
+    assert _resolve_v4_section_key(v4, "04-2-sub-1") is None
+
+
+def test_alias_resolver_miss_returns_none():
+    v4 = _fake_v4("99-1")
+    assert _resolve_v4_section_key(v4, "04-2-sub-1") is None
+    assert (
+        _resolve_v4_section_key(v4, "04-2-sub-1", alias_keys=["04-2.1"])
+        is None
+    )
+
+
+# ─── C. align_sections_to_v4_granularity ────────────────────────────────────
+
+
+def _section(section_id, num, title, raw_content):
+    """Build an MdxSection with default sub-section schema fields."""
+    return MdxSection(
+        section_id=section_id,
+        section_num=num,
+        title=title,
+        raw_content=raw_content,
+    )
+
+
+def test_mdx_section_default_construction_preserves_4_positional_callers():
+    # IMP-08 B-3 : MdxSection still accepts the legacy 4-positional shape
+    # (defaults for heading_number / v4_alias_keys / sub_sections).
+    s = MdxSection("04-1", 1, "1. Top", "body")
+    assert s.heading_number is None
+    assert s.v4_alias_keys == []
+    assert s.sub_sections == []
+
+
+def test_align_passthrough_when_v4_key_exact_match():
+    # Section already aligned to V4 key (no override target): aligner
+    # keeps it untouched. Parent-level V4 evidence flows via exact-match
+    # lookup.
+    sections = [_section("04-1", 1, "1. Top", "body")]
+    v4 = {"mdx_sections": {"04-1": {"judgments_full32": []}}}
+    out = align_sections_to_v4_granularity(sections, v4)
+    assert len(out) == 1
+    assert out[0].section_id == "04-1"
+
+
+def test_align_parent_v4_exact_keeps_section_when_no_override_targets_sub():
+    # Backward-compat axis: when V4 carries the parent exact key and no
+    # drag/drop override targets a sub-id of this section, the aligner
+    # MUST keep the parent (preserves V4 evidence at parent granularity).
+    raw = "### 2.1 First\nbody1\n### 2.2 Second\nbody2\n"
+    sections = [_section("03-2", 2, "2. Parent", raw)]
+    v4 = {"mdx_sections": {"03-2": {"judgments_full32": []}}}
+    out = align_sections_to_v4_granularity(sections, v4)
+    assert [s.section_id for s in out] == ["03-2"]
+
+
+def test_align_force_drills_when_override_targets_sub_id_with_parent_in_v4():
+    # Stage 5 R2 blocker-fix regression: when V4 has the parent exact key
+    # AND an override targets a sub-id of that section, the aligner MUST
+    # drill regardless of V4 parent presence. This makes drag/drop
+    # addressing deterministic across all V4 yaml shapes.
+    raw = "### 2.1 First\nbody1\n### 2.2 Second\nbody2\n"
+    sections = [_section("04-2", 2, "2. Parent", raw)]
+    v4 = {
+        "mdx_sections": {
+            "04-2": {"judgments_full32": []},      # parent V4 entry present
+            "04-2.1": {"judgments_full32": []},    # plus decimal sub entries
+            "04-2.2": {"judgments_full32": []},
+        }
+    }
+    out = align_sections_to_v4_granularity(
+        sections, v4, override_target_section_ids=["04-2-sub-1"]
+    )
+    # Force-drill: parent id MUST be replaced by canonical sub-ids.
+    assert [s.section_id for s in out] == ["04-2-sub-1", "04-2-sub-2"]
+    # Decimal aliases preserved (N-R5: decimal heading_number).
+    assert out[0].v4_alias_keys == ["04-2.1"]
+    assert out[1].v4_alias_keys == ["04-2.2"]
+
+
+def test_align_top_level_override_target_does_not_force_drill_other_sections():
+    # Top-level override target ("primary=03-1") has no derive_parent_id,
+    # so it MUST NOT force-drill any section. Only "X-sub-N" targets
+    # trigger force-drill on parent X.
+    raw = "### 2.1 First\nbody1\n"
+    sections = [
+        _section("03-1", 1, "1. Top", "body"),
+        _section("03-2", 2, "2. Parent", raw),
+    ]
+    v4 = {
+        "mdx_sections": {
+            "03-1": {"judgments_full32": []},
+            "03-2": {"judgments_full32": []},
+        }
+    }
+    out = align_sections_to_v4_granularity(
+        sections, v4, override_target_section_ids=["03-1"]
+    )
+    # No sub-id target -> both sections kept at parent granularity.
+    assert [s.section_id for s in out] == ["03-1", "03-2"]
+
+
+def test_align_drill_emits_canonical_ordinal_id_with_decimal_alias():
+    # Decimal H3 headings -> canonical ordinal id + decimal alias (legacy V4 key).
+    raw = "### 2.1 First\nbody1\n### 2.2 Second\nbody2\n"
+    sections = [_section("04-2", 2, "2. Parent", raw)]
+    v4 = {"mdx_sections": {}}  # forces drill (no exact key)
+    out = align_sections_to_v4_granularity(sections, v4)
+    assert [s.section_id for s in out] == ["04-2-sub-1", "04-2-sub-2"]
+    assert [s.heading_number for s in out] == ["2.1", "2.2"]
+    # N-R5 : decimal headings -> alias emitted.
+    assert out[0].v4_alias_keys == ["04-2.1"]
+    assert out[1].v4_alias_keys == ["04-2.2"]
+
+
+def test_align_drill_integer_only_h3_emits_no_alias_n_r5_guard():
+    # N-R5 : integer-only H3 (e.g., "### 1 Title") must NOT generate an alias,
+    # otherwise it would collide with sibling parent V4 entries (`{mdx_id}-1`).
+    raw = "### 1 Alpha\nbody1\n### 2 Beta\nbody2\n"
+    sections = [_section("05-2", 2, "2. Parent", raw)]
+    v4 = {"mdx_sections": {}}
+    out = align_sections_to_v4_granularity(sections, v4)
+    assert [s.section_id for s in out] == ["05-2-sub-1", "05-2-sub-2"]
+    assert [s.heading_number for s in out] == ["1", "2"]
+    assert out[0].v4_alias_keys == []
+    assert out[1].v4_alias_keys == []
+
+
+def test_align_drill_undecorated_h3_emits_no_alias():
+    # Plain `### Title` without numeric prefix -> heading_number=None, no alias.
+    raw = "### Alpha\nbody1\n### Beta\nbody2\n"
+    sections = [_section("03-3", 3, "3. Parent", raw)]
+    v4 = {"mdx_sections": {}}
+    out = align_sections_to_v4_granularity(sections, v4)
+    assert [s.section_id for s in out] == ["03-3-sub-1", "03-3-sub-2"]
+    assert [s.heading_number for s in out] == [None, None]
+    assert all(s.v4_alias_keys == [] for s in out)
+
+
+def test_align_no_h3_passes_section_through_unchanged():
+    # No H3 sub-headings in raw_content -> aligner keeps the section.
+    sections = [_section("04-1", 1, "1. Top", "no subheadings here\njust prose")]
+    v4 = {"mdx_sections": {}}
+    out = align_sections_to_v4_granularity(sections, v4)
+    assert len(out) == 1
+    assert out[0].section_id == "04-1"
+
+
+def test_align_resolver_round_trip_with_legacy_v4_alias():
+    # End-to-end : aligner emits canonical id + alias keys; resolver finds the
+    # legacy decimal key in V4 via alias path (no parent promotion).
+    raw = "### 2.1 First\nbody1\n"
+    sections = [_section("04-2", 2, "2. Parent", raw)]
+    v4 = {"mdx_sections": {"04-2.1": {"judgments_full32": []}}}
+    out = align_sections_to_v4_granularity(sections, v4)
+    sub = out[0]
+    assert sub.section_id == "04-2-sub-1"
+    resolved = _resolve_v4_section_key(
+        v4, sub.section_id, alias_keys=sub.v4_alias_keys
+    )
+    assert resolved == "04-2.1"