serialization: scene save/load (Unity-style YAML + UE-style GUID registry)

Engine-layer SceneSerializer (yaml-cpp submodule): entities/components
to versioned YAML, atomic clear-and-rebuild on load with per-entity
fault tolerance, missing-field defaults, unknown-component skip and
UUID dedup. Asset references are {Guid, Path} pairs resolved through
the new AssetManager GUID registry (assets/asset_registry.yaml) with
path fallback; procedural:// pseudo-paths rebuild from parameters.
Editor File menu gets Save/Load Scene modals; app re-resolves stale
entity handles via onSceneLoaded callback.

App/editor/EditorContext.h

@@ -40,6 +40,9 @@ struct EditorContext {
 
     std::function<int()> getEnvIndex; // 当前环境表下标
     std::function<void(int)> setEnvIndex;
+    // 场景加载成功后由 SandboxApp 重解析悬空句柄（registry 清场重建后
+    // 应用层持有的 EntityHandle 全部悬空，见 Scene::Clear 注释）
+    std::function<void()> onSceneLoaded;
 };
 
 #endif //HELLOVULKAN_EDITORCONTEXT_H