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_internal_region_planner.py

@@ -62,6 +62,7 @@ class InternalRegion:
                              — B2 v0 에서 kind="frame_match" / frame_id=None /
                                display_strategy="inline_full" 고정.
                                실제 frame 결정은 Step 9 / B4 책임.
+      source_shape_index   : positional index from B1 source_shape split (Option 1, optional)
     """
 
     region_id: str
@@ -70,6 +71,7 @@ class InternalRegion:
     ratio_estimate: float
     content_unit_ids: list[str]
     frame_match_strategy: dict
+    source_shape_index: Optional[int] = None
 
 
 @dataclass
@@ -146,6 +148,75 @@ _TYPE_ORDER_PRIORITY: dict[str, int] = {
 }
 
 
+# ─── Option 1 source_shape-aware planner ─────────────────────────
+
+
+def _plan_by_source_shape_index(
+    content_objects: list[ContentObject],
+    section_id: str,
+) -> ZoneRegionPlan:
+    """source_shape_index 기준 *positional* region grouping (Option 1).
+
+    같은 source_shape_index 끼리 1 region. mapper 의 split_source 와 cardinality align —
+    F13 의 top_bullets 3 개 → 3 region 으로 mapper pillar_1/2/3 와 1:1 positional.
+    """
+    groups: dict[int, list[ContentObject]] = {}
+    for obj in content_objects:
+        if obj.source_shape_index is None:
+            continue
+        groups.setdefault(obj.source_shape_index, []).append(obj)
+
+    sorted_indices = sorted(groups.keys())
+
+    # size proxy + ratio (positional region 내부 size_estimate 합산)
+    index_sizes: dict[int, float] = {idx: sum(_size_proxy(o) for o in groups[idx]) for idx in sorted_indices}
+    total_size = sum(index_sizes.values())
+    if total_size <= 0:
+        equal_share = 1.0 / max(len(sorted_indices), 1)
+        index_sizes = {idx: equal_share for idx in sorted_indices}
+        total_size = sum(index_sizes.values()) or 1.0
+
+    regions: list[InternalRegion] = []
+    for ord_idx, sidx in enumerate(sorted_indices, start=1):
+        objs = groups[sidx]
+        # role / content_type : group 내 첫 obj 의 type 기반 (Option 1 pilot = text_block 동질)
+        primary_obj = objs[0]
+        ctype = primary_obj.type
+        regions.append(
+            InternalRegion(
+                region_id=f"{section_id}.region-{ord_idx}",
+                role=_TYPE_ROLE.get(ctype, "primary"),
+                content_type=ctype,
+                ratio_estimate=round(index_sizes[sidx] / total_size, 4),
+                content_unit_ids=[o.id for o in objs],
+                frame_match_strategy={
+                    "kind": "frame_match",
+                    "frame_id": None,
+                    "display_strategy": "inline_full",
+                },
+                source_shape_index=sidx,
+            )
+        )
+
+    region_count = len(regions)
+    if region_count == 1:
+        layout_type = "region-single"
+        placement = "single"
+    else:
+        layout_type = "region-vertical-stack"
+        placement = "vertical"
+
+    region_order = [r.region_id for r in regions]
+    return ZoneRegionPlan(
+        internal_regions=regions,
+        region_layout=RegionLayout(
+            region_layout_type=layout_type,
+            region_order=region_order,
+            region_placement=placement,
+        ),
+    )
+
+
 # ─── Public entry ────────────────────────────────────────────────
 
 
@@ -186,6 +257,11 @@ def plan_internal_regions(
     if not content_objects:
         return ZoneRegionPlan()
 
+    # Option 1 source_shape-aware path : ContentObjects 가 source_shape_index 보유 시 *positional*
+    # grouping. 같은 index 끼리 1 region. mapper 의 split_source 와 cardinality align.
+    if any(o.source_shape_index is not None for o in content_objects):
+        return _plan_by_source_shape_index(content_objects, section_id)
+
     # 1. type 별 grouping
     groups = _group_by_type_preserving_order(content_objects)