feat(IMP-06): Stage 4 part 1 — replaced_auto_unit field + comment fix

Refs #6

Stage 4 split per Codex #10 acceptance: this commit lands the schema +
trace refinements required before the render-path rewiring. The actual
units/zones_data/Step 9/Step 20 plan-driven materialization remains in
Stage 4 part 2 (follow-up commit) so each commit is reviewable on its
own and regression-safe.

- _build_position_assignment_plan: add replaced_auto_unit field. Populated
  only when the explicitly overridden position already held an auto unit
  AND that auto unit had different source_section_ids than the override.
  Documents a same-position override replacement as a distinct audit fact,
  separate from skipped_collided_auto_units which captures cross-position
  whole-skips per the locked collision policy.
- Backfill replaced_auto_unit = None on the empty/collision/auto branches
  for schema-stable consumers.
- Update the override-application comment near the helper invocation so it
  no longer claims the helper "reorders units"; Stage 4 part 2 will be the
  commit that wires the plan into the actual render path.
- Helper unit tests: assert replaced_auto_unit shape in the collision
  scenario and add a dedicated case that distinguishes same-sections
  (template swap via --override-frame -> None) from different-sections
  same-position replacement (populated, reason="same_position_override_replacement").

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

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

src/phase_z2_pipeline.py

@@ -954,6 +954,18 @@ def _build_position_assignment_plan(
                 sid for sid in previous_source_section_ids if sid not in sids_set
             ]
             template_id, skipped_reason, selector_trace = _resolve_template_for_override(pos, sids)
+            # IMP-06 Stage 4 (Codex #9 R1 + Claude #9 Catch L + Codex #10) — replaced_auto_unit
+            # populated only when the same position previously had an auto unit and that
+            # auto unit was different from the requested override. Documents "this auto
+            # unit was removed from this position to apply the override" as a distinct
+            # audit fact (vs skipped_collided_auto_units which is cross-position skip).
+            replaced_auto_unit = None
+            if auto_unit is not None and list(auto_unit.source_section_ids) != list(sids):
+                replaced_auto_unit = {
+                    "unit_id": _unit_id(list(auto_unit.source_section_ids)),
+                    "source_section_ids": list(auto_unit.source_section_ids),
+                    "reason": "same_position_override_replacement",
+                }
             plan.append({
                 "position": pos,
                 "assignment_source": "cli_override",
@@ -967,6 +979,7 @@ def _build_position_assignment_plan(
                     "zone_id": pos,
                     "requested_section_ids": list(sids),
                 },
+                "replaced_auto_unit": replaced_auto_unit,
                 "skipped_collided_auto_units": [],
                 "uncovered_section_ids": uncovered_from_previous,
                 "skipped_reason": skipped_reason,
@@ -984,6 +997,7 @@ def _build_position_assignment_plan(
                     "template_id": None,
                     "previous_source_section_ids": [],
                     "section_assignment_override": None,
+                    "replaced_auto_unit": None,
                     "skipped_collided_auto_units": [],
                     "uncovered_section_ids": [],
                     "skipped_reason": "no_auto_unit_available",
@@ -1008,6 +1022,7 @@ def _build_position_assignment_plan(
                     "template_id": None,
                     "previous_source_section_ids": list(auto_unit.source_section_ids),
                     "section_assignment_override": None,
+                    "replaced_auto_unit": None,
                     "skipped_collided_auto_units": [{
                         "unit_id": unit_id_str,
                         "source_section_ids": list(auto_unit.source_section_ids),
@@ -1029,6 +1044,7 @@ def _build_position_assignment_plan(
                     ),
                     "previous_source_section_ids": list(auto_unit.source_section_ids),
                     "section_assignment_override": None,
+                    "replaced_auto_unit": None,
                     "skipped_collided_auto_units": [],
                     "uncovered_section_ids": [],
                     "skipped_reason": None,
@@ -2042,11 +2058,12 @@ def run_phase_z2_mvp1(
         layout_preset = override_layout
         layout_override_applied = True
 
-    # IMP-06 (#6 / Codex #6,#7 15-axis lock) — zone-section assignment override.
+    # IMP-06 (#6 / Codex #6/#7/#10 lock) — zone-section assignment override.
     # Applied AFTER final layout_preset resolution. ZONE_ID = layout positions.
-    # The helper computes a position_assignment_plan; we then validate unknown
-    # zone ids / unknown section ids and reorder units to match the plan so that
-    # downstream zones_data / debug_zones / Step 9 derive consistently.
+    # 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).
     section_assignment_plan: Optional[list[dict]] = None
     section_assignment_summary: Optional[dict] = None
     if override_section_assignments and layout_preset is not None: