feat: parent-child transform hierarchy (UE Actor attach semantics)

- TransformComponent TRS = local (relative to parent); cached world SRT
  composed hierarchically (parent pos/rot/scale, no matrix decomposition
  shear under non-uniform parent scale)
- HierarchyComponent (parent + children, lazily attached); Scene::
  SetParent (keepWorld, cycle-safe) / GetParent / IsDescendantOf /
  SetWorldPose (world->local inverse, used by physics writeback & gizmo)
- UpdateTransforms: DFS from roots, recompose only dirty-or-dirty-
  ancestor subtrees (parents before children)
- DestroyEntity: descendants buried with parent (UE cascade)
- Serialization v2: optional Parent uuid key, two-pass relink on load
- Hierarchy panel: tree view, drag-drop reparent / drop-to-root detach,
  Create Child / Detach context menu; Inspector shows parent name
- Consumers moved to world-space reads: lights, camera pos, physics
  bodies/characters/joints/collider baking, gizmo, F-focus, AlignTo

App/editor/StatsPanel.cpp

@@ -32,7 +32,8 @@ void StatsPanel::Draw(const EditorContext& ctx)
             Engine::ECS::TransformComponent* camT = nullptr;
             if (ctx.scene->GetPrimaryCamera(&camT) != nullptr && camT != nullptr)
             {
-                const glm::vec3& pos = camT->GetPosition();
+                // world 位姿（相机实体可能挂父层级下）
+                const glm::vec3& pos = camT->GetWorldPosition();
                 ImGui::Text("Game camera pos");
                 ImGui::Text("(%.3f, %.3f, %.3f)", pos.x, pos.y, pos.z);
             }