feat(IMP-06): Stage 4 Part 2 — render-path integration (units rebuild + empty zone + Catch K fix)

Refs #6

After `position_assignment_plan` is built, rebuild the `units` list to be
plan-aligned so downstream `zones_data` / `debug_zones` / mapper / render
all see the post-override sequence. This resolves the long-standing
trace-only gap and closes Codex Catch K naturally because the helper now
actually drives downstream materialization.

- run_phase_z2_mvp1: after `_build_position_assignment_plan`, rebuild
  `units` ordered by `position_assignment_plan`. `cli_override` entries
  synthesize a CompositionUnit (resolved template_id + concatenated
  section raw_content + contract frame_id + selection_path="cli_override"
  + override audit in rationale). `auto` entries reuse the original
  planner unit. empty/collision-skipped entries become None placeholders
  so the downstream zone loop can emit an explicit empty zone record
  without distorting layout allocation.
- zones_data / debug_zones loop: handle `unit is None` by appending an
  explicit empty record with template_id="__empty__", content_weight=0,
  min_height_px=0, plus the plan's skipped_reason / replaced_auto_unit /
  skipped_collided_auto_units / uncovered_section_ids audit fields.
- partial render loop: `template_id == "__empty__"` short-circuits to
  `partial_html = ""` so the slide_base zones loop preserves grid identity
  without raising TemplateNotFound.
- Update the helper-invocation comment so it now describes the actual
  Part 2 behavior (units rebuild + empty zone handling). Catch K is no
  longer a future-tense placeholder.

Stage 4 Part 3 (follow-up commit) will add: Step 9 application_plan
plan-aware additive fields, Step 20 list-shaped filtered_section_reasons
entries for override-uncovered sections (Codex #10 Catch O schema), and
integration tests proving zones_data["top"] actually contains the
overridden section ids when --override-section-assignment is supplied.

Regression: 20/20 unit tests pass (9 IMP-06 helper + 8 IMP-05 fallback +
2 catalog invariant + 1 dedicated replaced_auto_unit test), smoke
self-check 11/11 (IMP-04 F17 calibration intact).

No AI, no calculate_fit, no full planner rerun, no frontend, no sample
hardcoding. plan_composition() signature preserved.

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

src/phase_z2_pipeline.py

@@ -1299,6 +1299,12 @@ def render_slide(slide_title: str, slide_footer: Optional[str],
         autoescape=select_autoescape(["html"]),
     )
     for zone in zones_data:
+        # Stage 4 Part 2 (Codex #10 Catch N) — empty zone produced by section
+        # assignment override has no partial template; render an empty string so
+        # the slide_base zones loop preserves grid identity without TemplateNotFound.
+        if zone.get("template_id") == "__empty__":
+            zone["partial_html"] = ""
+            continue
         partial = env.get_template(f"families/{zone['template_id']}.html")
         zone["partial_html"] = partial.render(slot_payload=zone["slot_payload"])
 
@@ -2058,12 +2064,16 @@ def run_phase_z2_mvp1(
         layout_preset = override_layout
         layout_override_applied = True
 
-    # IMP-06 (#6 / Codex #6/#7/#10 lock) — zone-section assignment override.
+    # IMP-06 (#6 / Codex #6/#7/#10/#11/#12 lock) — zone-section assignment override.
     # Applied AFTER final layout_preset resolution. ZONE_ID = layout positions.
     # The helper validates unknown zone ids / unknown section ids and builds a
-    # `position_assignment_plan`. The plan drives downstream zones_data /
-    # debug_zones / Step 9 application_plan / Step 20 coverage (see Stage 4
-    # block further below where `units_by_position` is materialized).
+    # `position_assignment_plan`. Immediately below, Stage 4 Part 2 rebuilds the
+    # `units` list aligned with that plan: cli_override entries synthesize a
+    # CompositionUnit, auto entries reuse the original planner unit, and empty/
+    # collision-skipped entries become None placeholders. The downstream
+    # zones_data / debug_zones loop then handles None entries by emitting an
+    # explicit empty zone record (template_id="__empty__") so the slide grid
+    # preserves position identity without distorting layout allocation.
     section_assignment_plan: Optional[list[dict]] = None
     section_assignment_summary: Optional[dict] = None
     if override_section_assignments and layout_preset is not None:
@@ -2106,6 +2116,61 @@ def run_phase_z2_mvp1(
             file=sys.stderr,
         )
 
+        # Stage 4 Part 2 — rebuild units list aligned with position_assignment_plan so
+        # downstream zones_data / debug_zones / Step 9 / Step 20 derive from the plan
+        # rather than the original auto-selected sequence. None placeholders preserve
+        # position identity for empty/collision-skipped zones (Codex #10 Catch N).
+        from src.phase_z2_composition import CompositionUnit
+        plan_units: list = []
+        for entry in section_assignment_plan:
+            assignment_source = entry["assignment_source"]
+            if assignment_source == "cli_override" and entry["template_id"] is not None:
+                sids = entry["source_section_ids"]
+                raw_content_parts = []
+                title_parts = []
+                for sid in sids:
+                    sect = sections_by_id.get(sid)
+                    if sect is None:
+                        continue
+                    raw_content_parts.append(sect.raw_content or "")
+                    if sect.title:
+                        title_parts.append(sect.title)
+                contract = get_contract(entry["template_id"])
+                contract_frame_id = (contract or {}).get("frame_id") or ""
+                override_unit = CompositionUnit(
+                    source_section_ids=list(sids),
+                    merge_type="cli_override",
+                    frame_template_id=entry["template_id"],
+                    frame_id=str(contract_frame_id),
+                    frame_number=0,
+                    confidence=0.0,
+                    label="use_as_is",
+                    phase_z_status="matched_zone",
+                    raw_content="\n\n".join(raw_content_parts),
+                    title=" / ".join(title_parts) if title_parts else "+".join(sids),
+                    v4_rank=None,
+                    selection_path="cli_override",
+                    fallback_reason=None,
+                    score=0.0,
+                    rationale={
+                        "section_assignment_override": entry["section_assignment_override"],
+                        "replaced_auto_unit": entry["replaced_auto_unit"],
+                    },
+                )
+                plan_units.append(override_unit)
+            elif assignment_source == "auto":
+                # Find original auto unit by source_section_ids
+                matched = None
+                for u in units:
+                    if list(u.source_section_ids) == entry["source_section_ids"]:
+                        matched = u
+                        break
+                plan_units.append(matched)  # may be None if not found (unexpected)
+            else:
+                # empty / collision-skipped
+                plan_units.append(None)
+        units = plan_units
+
     if not units or layout_preset is None:
         # composition planner 결과 = 0 units. Sections 가 모두 V4 lookup 실패 또는
         # status filter 통과 못 함. error.json 기록 후 abort.
@@ -2254,6 +2319,53 @@ def run_phase_z2_mvp1(
 
     for i, unit in enumerate(units):
         position = positions[i] if i < len(positions) else f"zone_{i}"
+        # Stage 4 Part 2 — empty position (override plan produced no renderable unit).
+        # Preserve zone identity with an explicit empty record so layout/grid stay
+        # structurally consistent without distorting allocation (Codex #10 Catch N).
+        if unit is None:
+            plan_entry = None
+            if section_assignment_plan is not None and i < len(section_assignment_plan):
+                plan_entry = section_assignment_plan[i]
+            zones_data.append({
+                "position": position,
+                "template_id": "__empty__",
+                "slot_payload": {},
+                "content_weight": {"score": 0},
+                "min_height_px": 0,
+                "assignment_source": "empty",
+                "skipped_reason": (
+                    (plan_entry or {}).get("skipped_reason")
+                    or "section_assignment_override_empty_or_unrenderable"
+                ),
+            })
+            debug_zones.append({
+                "position": position,
+                "source_section_ids": [],
+                "merge_type": "empty",
+                "title": "",
+                "v4_template_id": "__empty__",
+                "v4_label": None,
+                "v4_confidence": 0.0,
+                "selection_path": "section_assignment_empty",
+                "fallback_reason": None,
+                "fallback_used": False,
+                "phase_z_status": None,
+                "composition_score": 0.0,
+                "composition_rationale": {},
+                "composition_notes": [],
+                "mapper_type": "empty",
+                "contract_id": "__empty__",
+                "contract_frame_id": None,
+                "assignment_source": "empty",
+                "skipped_reason": (
+                    (plan_entry or {}).get("skipped_reason")
+                    or "section_assignment_override_empty_or_unrenderable"
+                ),
+                "replaced_auto_unit": (plan_entry or {}).get("replaced_auto_unit"),
+                "skipped_collided_auto_units": (plan_entry or {}).get("skipped_collided_auto_units", []),
+                "uncovered_section_ids": (plan_entry or {}).get("uncovered_section_ids", []),
+            })
+            continue
         synth_section = MdxSection(
             section_id="+".join(unit.source_section_ids),
             section_num=0,