Add Phase Z B4 source-shape-aware placement

- enable B1/B2/B4 source-shape-aware F13 placement behind env flag
- align F13 placement_trace with mapper top_bullets cardinality
- preserve canonical render output when flag is off

src/phase_z2_pipeline.py

@@ -1063,7 +1063,17 @@ def run_phase_z2_mvp1(mdx_path: Path, run_id: Optional[str] = None) -> Path:
         # 결과 (PlacementPlan) = debug_zones[i].placement_trace 로 *기록만*.
         # render path / mapper output / final.html 모두 미변경 — B5 baseline SHA 유지.
         # B4 frame selection = catalog declaration order (V4 evidence 미사용 — 별 axis).
-        content_objects = extract_content_objects(synth_section)
+        # Option 1 (PHASE_Z_B4_SOURCE_SHAPE_ENABLED, default OFF) : pilot = F13 top_bullets only.
+        b4_source_shape_enabled = (
+            os.environ.get("PHASE_Z_B4_SOURCE_SHAPE_ENABLED", "").strip().lower()
+            in {"1", "true", "yes"}
+        )
+        b1_source_shape = (
+            contract.get("source_shape")
+            if b4_source_shape_enabled and contract.get("source_shape") == "top_bullets"
+            else None
+        )
+        content_objects = extract_content_objects(synth_section, source_shape=b1_source_shape)
         placement_plan = plan_placement(
             content_objects=content_objects,
             frame_contracts=list(load_frame_contracts().values()),