feat(IMP-08): U3 — frontend wire (zoneSections override)

Wires the frontend drag/drop zone assignment through to the backend
--override-section-assignment CLI flag.

PipelineOverrides gains an optional zoneSections field
(Record<string, string[]>) carrying canonical ordinal section ids
(e.g., "top": ["04-2-sub-1"]).

Vite middleware /api/run accepts overrides.zoneSections and forwards
each non-empty zone as `--override-section-assignment ZONE=sid[,sid]`.
Empty arrays and non-string sids are filtered to avoid bogus
assignments from a partially-built UI state.

Home.tsx builds the override with a diff-vs-default guard per Codex
Stage 3 R3 B3 fix : createInitialUserSelection seeds zone_sections with
the auto plan, so a literal copy would pollute backend assignment-source
provenance even on a fresh re-render. The diff compares each zone's
section list against sourcePlan.zones[].section_ids and only emits zones
that differ. Toast summary now reports zoneSections=N when forwarded.

Smoke verification : python -m src.phase_z2_pipeline samples/mdx_batch/04.mdx
test_imp08_smoke --override-section-assignment primary=04-2-sub-1 produces
section_assignment_plan with assignment_source=cli_override and
v4_selector_trace.candidates populated via the U1 alias resolver
(04-2-sub-1 -> 04-2.1 V4 entry).

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

src/phase_z2_pipeline.py

@@ -31,7 +31,7 @@ import re
 import shutil
 import sys
 import time
-from dataclasses import asdict, dataclass
+from dataclasses import asdict, dataclass, field
 from pathlib import Path
 from typing import Optional
 
@@ -136,6 +136,13 @@ class MdxSection:
     section_num: int
     title: str
     raw_content: str
+    # IMP-08 B-3 sub-section schema (additive, defaults preserve 4-positional callers).
+    # heading_number: decimal "2.1" from MDX `### 2.1 Title` capture (U2-populated).
+    # v4_alias_keys: legacy V4 keys to try when canonical ordinal id misses (e.g. "04-2.1").
+    # sub_sections: raw child payloads from section_parser (Stage 0 adapter consumes).
+    heading_number: Optional[str] = None
+    v4_alias_keys: list = field(default_factory=list)
+    sub_sections: list = field(default_factory=list)
 
 
 @dataclass
@@ -368,6 +375,16 @@ def load_v4_result() -> dict:
 def align_sections_to_v4_granularity(sections: list[MdxSection], v4: dict) -> list[MdxSection]:
     """V4 section granularity 에 맞춰 sections 조정.
 
+    IMP-08 B-3 : canonical sub-section id ``${section_id}-sub-${ordinal}``
+    (예 : ``04-2-sub-1``) 를 emit 하고, legacy V4 키 (``04-2.1``) 는
+    ``v4_alias_keys`` 로 보존하여 ``_resolve_v4_section_key`` 가 alias 경로로
+    매칭한다. canonical ordinal id 는 frontend drag/drop override 와 동일
+    schema (`section_id-sub-N`).
+
+    N-R5 alias guard : heading_number 가 decimal (``2.1``) 일 때만 alias
+    emit. integer-only (``1``) / non-numeric heading 은 alias 0 — sibling
+    parent V4 evidence 로 잘못 promote 되는 collision 방지 (RULE 0).
+
     각 section 에 대해 :
       - V4 에 section.section_id 키 있음 → 그대로 유지 (## level 매칭)
       - V4 에 키 없고 raw_content 에 ### sub-section 존재 → ### 로 drill
@@ -381,31 +398,52 @@ def align_sections_to_v4_granularity(sections: list[MdxSection], v4: dict) -> li
     v4_keys = set(v4.get("mdx_sections", {}).keys())
     aligned: list[MdxSection] = []
 
+    # IMP-08 B-3 : capture optional heading-number prefix (decimal "2.1" or
+    # integer "1") + heading title. None group = bare "### Title".
+    sub_pattern = re.compile(
+        r"^###\s+(?:(\d+(?:\.\d+)?)\s+)?(.+?)$", re.MULTILINE
+    )
+    decimal_re = re.compile(r"\d+\.\d+")
+
     for section in sections:
         if section.section_id in v4_keys:
             aligned.append(section)
             continue
 
-        # ### drill 시도
-        sub_pattern = re.compile(r"^###\s+(\d+\.\d+)\s+(.+?)$", re.MULTILINE)
         sub_matches = list(sub_pattern.finditer(section.raw_content))
         if not sub_matches:
             aligned.append(section)  # drill 불가, V4 lookup 에서 abort 됨
             continue
 
-        # ### sub-section 추출
         mdx_id = section.section_id.split("-")[0]   # e.g., "04"
-        for i, m in enumerate(sub_matches):
-            subnum = m.group(1)                       # e.g., "2.1"
+        for ordinal, m in enumerate(sub_matches, start=1):
+            heading_number = m.group(1)  # decimal "2.1" / integer "1" / None
             sub_title = m.group(2).strip()
             start = m.end()
-            end = sub_matches[i + 1].start() if i + 1 < len(sub_matches) else len(section.raw_content)
+            end = (
+                sub_matches[ordinal].start()
+                if ordinal < len(sub_matches)
+                else len(section.raw_content)
+            )
             raw = section.raw_content[start:end].strip()
+
+            # N-R5 : alias only for decimal heading numbers. integer-only
+            # H3 (`### 1`) or undecorated H3 produce no alias to avoid
+            # sibling-parent V4 collisions (e.g., 05.mdx integer H3s).
+            alias_keys: list[str] = []
+            if heading_number and decimal_re.fullmatch(heading_number):
+                alias_keys.append(f"{mdx_id}-{heading_number}")
+
+            title = (
+                f"{heading_number} {sub_title}" if heading_number else sub_title
+            )
             aligned.append(MdxSection(
-                section_id=f"{mdx_id}-{subnum}",      # e.g., "04-2.1"
+                section_id=f"{section.section_id}-sub-{ordinal}",
                 section_num=section.section_num,
-                title=f"{subnum} {sub_title}",
+                title=title,
                 raw_content=raw,
+                heading_number=heading_number,
+                v4_alias_keys=alias_keys,
             ))
 
     return aligned
@@ -424,8 +462,39 @@ def _v4_match_from_judgment(section_id: str, judgment: dict, rank: Optional[int]
     )
 
 
-def lookup_v4_match(v4: dict, section_id: str) -> Optional[V4Match]:
-    sec = v4.get("mdx_sections", {}).get(section_id)
+def _resolve_v4_section_key(
+    v4: dict,
+    section_id: str,
+    *,
+    alias_keys: Optional[list] = None,
+) -> Optional[str]:
+    """Resolve a V4 ``mdx_sections`` key for *section_id*.
+
+    Resolution order :
+      1. exact match (canonical ordinal id wins)
+      2. alias_keys in given order (e.g. legacy decimal ``04-2.1`` for ``04-2-sub-1``)
+      3. None on miss.
+
+    Never promotes to parent or sibling — that would reinterpret V4 evidence
+    (axis 7 hybrid lock, RULE 0). U1 callers pass alias_keys=None so the
+    function is byte-identical to the previous exact-match lookup; U2 populates
+    aliases from MDX heading_number metadata.
+    """
+    keys = v4.get("mdx_sections", {})
+    if section_id in keys:
+        return section_id
+    if alias_keys:
+        for a in alias_keys:
+            if a and a in keys:
+                return a
+    return None
+
+
+def lookup_v4_match(
+    v4: dict, section_id: str, *, alias_keys: Optional[list] = None
+) -> Optional[V4Match]:
+    resolved = _resolve_v4_section_key(v4, section_id, alias_keys=alias_keys)
+    sec = v4.get("mdx_sections", {}).get(resolved) if resolved else None
     if not sec:
         return None
     judgments = sec.get("judgments_full32", [])
@@ -462,13 +531,15 @@ def lookup_v4_match_with_fallback(
     *,
     raw_content: Optional[str] = None,
     max_rank: int = 3,
+    alias_keys: Optional[list] = None,
 ) -> tuple[Optional[V4Match], dict]:
     """Select V4 rank-1, or promote rank-2/3 when rank-1 is not auto-renderable.
 
     This is an IMP-05 selector only. It uses existing V4 labels, frame-contract
     presence, and the Phase Z capacity precheck; it does not call calculate_fit.
     """
-    sec = v4.get("mdx_sections", {}).get(section_id)
+    resolved = _resolve_v4_section_key(v4, section_id, alias_keys=alias_keys)
+    sec = v4.get("mdx_sections", {}).get(resolved) if resolved else None
     trace = {
         "section_id": section_id,
         "max_rank": max_rank,
@@ -571,7 +642,9 @@ def lookup_v4_match_with_fallback(
     return None, trace
 
 
-def lookup_v4_all_judgments(v4: dict, section_id: str) -> list[V4Match]:
+def lookup_v4_all_judgments(
+    v4: dict, section_id: str, *, alias_keys: Optional[list] = None
+) -> list[V4Match]:
     """V4 raw 32 entry 그대로 반환 — reject 포함, max_n filter 없음.
 
     Step 7-A axis 보강 (사용자 lock 2026-05-08) — 사용자 UI 가 모든 frame 의
@@ -581,7 +654,8 @@ def lookup_v4_all_judgments(v4: dict, section_id: str) -> list[V4Match]:
     Returns :
       list[V4Match] — 0~32 길이. raw judgments_full32 순서 (= V4 score desc) 보존.
     """
-    sec = v4.get("mdx_sections", {}).get(section_id)
+    resolved = _resolve_v4_section_key(v4, section_id, alias_keys=alias_keys)
+    sec = v4.get("mdx_sections", {}).get(resolved) if resolved else None
     if not sec:
         return []
     judgments = sec.get("judgments_full32", [])
@@ -592,7 +666,11 @@ def lookup_v4_all_judgments(v4: dict, section_id: str) -> list[V4Match]:
 
 
 def lookup_v4_candidates(
-    v4: dict, section_id: str, max_n: int = 6
+    v4: dict,
+    section_id: str,
+    max_n: int = 6,
+    *,
+    alias_keys: Optional[list] = None,
 ) -> list[V4Match]:
     """V4 non-reject 후보 list 반환 (Step 5 보완 axis — 사용자 lock 2026-05-08).
 
@@ -612,7 +690,8 @@ def lookup_v4_candidates(
       호출처 무변. 본 함수는 Step 5 artifact + Step 9 application_plan input
       위한 새 entry point.
     """
-    sec = v4.get("mdx_sections", {}).get(section_id)
+    resolved = _resolve_v4_section_key(v4, section_id, alias_keys=alias_keys)
+    sec = v4.get("mdx_sections", {}).get(resolved) if resolved else None
     if not sec:
         return []
     judgments = sec.get("judgments_full32", [])
@@ -932,7 +1011,12 @@ def _build_position_assignment_plan(
             if v4 is None or section is None:
                 return None, "no_v4_section", None
             raw_content = getattr(section, "raw_content", None)
-            match, trace = lookup_v4_match_with_fallback(v4, sid, raw_content=raw_content)
+            # IMP-08 B-3 : forward sub-section V4 aliases (decimal heading_number)
+            # when canonical ordinal id misses; safe for top-level sids (empty list).
+            alias_keys = list(getattr(section, "v4_alias_keys", []) or [])
+            match, trace = lookup_v4_match_with_fallback(
+                v4, sid, raw_content=raw_content, alias_keys=alias_keys
+            )
             if match is None:
                 return None, "no_direct_render_template", trace
             return match.template_id, None, trace
@@ -2039,6 +2123,11 @@ def run_phase_z2_mvp1(
     #    candidate (separate / parent_merged) → score → greedy non-overlapping select →
     #    layout preset (count-based v0).
     section_content_by_id = {s.section_id: s.raw_content for s in sections}
+    # IMP-08 B-3 : sub-section ordinal id -> legacy V4 key aliases (e.g. "04-2.1").
+    # Empty list for canonical (top-level) sections — U1 baseline path is exact-only.
+    section_alias_by_id: dict[str, list] = {
+        s.section_id: list(getattr(s, "v4_alias_keys", []) or []) for s in sections
+    }
     v4_fallback_traces: dict[str, dict] = {}
 
     def lookup_fn(sid: str) -> Optional[V4Match]:
@@ -2047,6 +2136,7 @@ def run_phase_z2_mvp1(
             sid,
             raw_content=section_content_by_id.get(sid),
             max_rank=3,
+            alias_keys=section_alias_by_id.get(sid),
         )
         v4_fallback_traces[sid] = trace
         return match
@@ -2054,7 +2144,7 @@ def run_phase_z2_mvp1(
     # Step 6-A axis (사용자 lock 2026-05-08) — V4 raw dict 흡수 fn.
     # composition module 은 V4 yaml shape 모름. 본 fn 만 통해 후보 list 받음.
     def candidates_lookup_fn(sid: str) -> list[V4Match]:
-        return lookup_v4_candidates(v4, sid)
+        return lookup_v4_candidates(v4, sid, alias_keys=section_alias_by_id.get(sid))
 
     units, layout_preset, comp_debug = plan_composition(
         sections, lookup_fn, V4_LABEL_TO_PHASE_Z_STATUS, MVP1_ALLOWED_STATUSES,
@@ -2777,6 +2867,11 @@ def run_phase_z2_mvp1(
         note="V4 evidence 와 B4 통합 미완 — 별 axis. 현재 = composition planner 의 V4 rank-1 채택.",
     )
     # Step 9 HTML — V4 top candidates per zone (rank 1~4)
+    # IMP-08 N-R6 diagnostic exemption : this report path is post-decision
+    # reporting only. Runtime selection goes through _resolve_v4_section_key
+    # (4 sites). Direct dict lookup here is intentional — debug_zones carries
+    # dict-shape entries without v4_alias_keys plumbing, and a miss here only
+    # yields a "V4 entry 없음" report line (runtime impact zero).
     try:
         with open(V4_RESULT_PATH, encoding="utf-8") as _vf:
             _v4_full = yaml.safe_load(_vf)
@@ -3263,7 +3358,12 @@ def run_phase_z2_mvp1(
         # 모든 frame 의 png 를 카드로 보여주기 위함).
         # unit_id = source_section_ids join. parent_merged 는 첫 section 의
         # judgments 사용 (parent V4 entry 가 그 section 에 있으므로).
-        v4_all_for_unit = lookup_v4_all_judgments(v4, unit.source_section_ids[0])
+        # IMP-08 B-3 : forward sub-section V4 aliases (decimal heading_number)
+        # when canonical ordinal id misses; U1 default = empty list (no change).
+        _first_sid = unit.source_section_ids[0]
+        v4_all_for_unit = lookup_v4_all_judgments(
+            v4, _first_sid, alias_keys=section_alias_by_id.get(_first_sid)
+        )
 
         # application_candidates : V4 후보 zip 으로 application_mode 변환
         app_candidates = []

tests/test_phase_z2_subsection_schema.py

@@ -0,0 +1,180 @@
+"""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 — aligner keeps it untouched.
+    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_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"