feat(engine): 支持多实体 body 与 selector 持久化

- 多实体主体拆分为独立 body_id，支持前缀匹配解析
- 重建结果新增 solid_count 字段
- fillet/chamfer 后 selector stable_id 可解析到演化后继

backend/engine/cdsl_engine/runtime.py

@@ -74,6 +74,7 @@ class GeometryAdapter(Protocol):
     """
 
     def topology_records(self, body: Any, feature_id: str, body_id: str) -> list[TopologyRecord]: ...
+    def body_solids(self, body: Any) -> list[Any]: ...
     def body_geometry(self, body: Any) -> dict[str, Any]: ...
     def faces_for_sketch(self, sketch: dict[str, Any]) -> list[Any]: ...
     def extrude(self, face: Any, direction: Vector3) -> Any: ...
@@ -108,9 +109,23 @@ class ExecutionSession:
     active_feature_id: str = ""
 
     def register_body(self, feature_id: str, body: Any, *, replay_node: FeaturePlanNode | None = None) -> None:
+        # #7 multi-body：主体可能是 Compound（多个独立实体，例如两个不相交的
+        # 拉伸）。body_id 现在反映真实实体结构而不是"最后一个特征的 id"：
+        # 每个独立 Solid 一个 body:{feature}:{index}，供 selector 精确匹配目标
+        # 实体；单体保持 body:{feature}（与历史行为完全一致）。
         self.body = body
         self.body_id = f"body:{feature_id}"
-        self.topology.replace_body_topology(feature_id, self.body_id, self.adapter.topology_records(body, feature_id, self.body_id))
+        solids = self.adapter.body_solids(body)
+        if len(solids) <= 1:
+            self.topology.replace_body_topology(feature_id, self.body_id, self.adapter.topology_records(body, feature_id, self.body_id))
+        else:
+            for index, solid in enumerate(solids):
+                member_id = f"{self.body_id}:{index}"
+                self.topology.replace_body_topology(
+                    feature_id, member_id,
+                    self.adapter.topology_records(solid, feature_id, member_id),
+                    active_body_id=self.body_id,
+                )
         self.topology.register(TopologyRecord(
             record_id=self.body_id, kind="body", feature_id=feature_id, body_id=self.body_id,
             geometry=self.adapter.body_geometry(body), value=body, owner_feature_ids=(feature_id,),
@@ -967,6 +982,9 @@ def rebuild_cdsl(cdsl: dict[str, Any], out_step: Path, *, strict: bool = True) -
         "out_step": str(out_step),
         "volume_mm3": float(geometry["volume_mm3"]),
         "bbox_mm": {"min": bbox[:3], "max": bbox[3:]},
+        # #7 multi-body：重建结果里的独立实体数（Compound 成员数），
+        # 与 batch 验证的 document_truth.geometry.solid_body_count 对齐。
+        "solid_count": len(session.adapter.body_solids(session.body)),
         "feature_results": [result.as_dict() for result in session.results.values()],
         "runtime_diagnostics": [diagnostic.as_dict() for diagnostic in diagnostics],
         "topology_records": [record.public_dict() for record in session.topology.records()],