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

Engine/src/Serialization/SceneSerializer.cpp

@@ -34,6 +34,7 @@
 #include <fstream>
 #include <string>
 #include <string_view>
+#include <unordered_map>
 #include <unordered_set>
 #include <vector>
 
@@ -176,6 +177,7 @@ namespace Engine::Serialization {
         {
             u64 uuid = 0;
             std::string name = "entity";
+            u64 parentUuid = 0;   // 0 = 根实体（层级键缺省）
             YAML::Node comps;   // 缺省 = Null（无组件）
         };
 
@@ -190,6 +192,13 @@ namespace Engine::Serialization {
             if (const auto* name = reg.try_get<ECS::NameComponent>(e)) {
                 out << YAML::Key << "Name" << YAML::Value << name->name;
             }
+            // 层级（UE Actor attach）：父存 UUID（跨存档稳定）；
+            // 悬空父（不该发生）按根实体写
+            if (const auto* h = reg.try_get<ECS::HierarchyComponent>(e);
+                h && h->parent != entt::null && reg.valid(h->parent)) {
+                out << YAML::Key << "Parent" << YAML::Value
+                    << reg.get<ECS::IdComponent>(h->parent).uuid.Value();
+            }
 
             out << YAML::Key << "Components" << YAML::Value << YAML::BeginMap;
 
@@ -524,6 +533,7 @@ namespace Engine::Serialization {
                     ParsedEntity pe;
                     pe.uuid = e["Entity"].as<u64>(0);
                     pe.name = e["Name"].as<std::string>(pe.name);
+                    pe.parentUuid = e["Parent"].as<u64>(0);
                     if (const YAML::Node comps = e["Components"];
                         comps && comps.IsMap()) {
                         // 仅在 rhs 有效时赋值（僵尸节点 operator= 会抛）
@@ -543,6 +553,7 @@ namespace Engine::Serialization {
         scene.Clear();
 
         std::unordered_set<u64> seenUuids;
+        std::unordered_map<u64, entt::entity> byUuid;   // 第二遍重链用
         for (ParsedEntity& pe : parsed) {
             auto handle = scene.CreateEntity(pe.name);
 
@@ -552,6 +563,8 @@ namespace Engine::Serialization {
                 uuid = UUID::New().Value();
             }
             handle.GetComponent<ECS::IdComponent>().uuid = UUID{uuid};
+            pe.uuid = uuid;   // 回写最终 uuid（重生成后父引用仍可命中）
+            byUuid.emplace(uuid, static_cast<entt::entity>(handle));
 
             const YAML::Node comps = pe.comps;   // 拷贝初始化，缺省 Null
 
@@ -734,6 +747,22 @@ namespace Engine::Serialization {
             }
         }
 
+        // ---- 阶段 3：重链父子（两遍解析：父可能出现在子之后）。
+        // keepWorld=false：local TRS 已从文件就位（相对父语义），
+        // 只建链标脏。防环由 SetParent 兜底（手编档成环 → 告警 + 挂根）
+        for (const ParsedEntity& pe : parsed) {
+            if (pe.parentUuid == 0) continue;
+            const auto c = byUuid.find(pe.uuid);
+            const auto p = byUuid.find(pe.parentUuid);
+            if (c == byUuid.end() || p == byUuid.end()) {
+                ENGINE_WARN("SceneSerializer: parent {} missing for entity "
+                            "{}, kept as root",
+                            pe.parentUuid, pe.uuid);
+                continue;
+            }
+            scene.SetParent(c->second, p->second, /*keepWorld=*/false);
+        }
+
         // 加载在 OnImGuiRender 段，错过本帧 UpdateTransforms：
         // 显式补算，否则首帧 World = 单位阵
         scene.UpdateTransforms();