feat: Play-In-Editor (PIE) with in-memory scene snapshot

Editor toolbar gains Play/Stop (Ctrl+P) with UE semantics:

- Play: serialize current scene to an in-memory YAML snapshot via new
  SceneSerializer::SerializeToString, switch display to kDisplayDirect
  (primary camera entity renders), unfreeze time (timeScale 0 -> 1)
- Stop: DeserializeFromString atomically rebuilds the pre-play scene,
  back to kDisplayViewport (editor camera), refreeze time; selection
  cleared and gizmo drag reset since all entity handles dangle
- SceneSerializer refactored: SerializeDoc/ParseRoot shared by file
  and memory variants; file Save/Load behavior unchanged
- Running state disables gizmo + editor hotkeys in the viewport (input
  goes to game scripts), draws yellow PLAYING hint and a no-camera
  warning; File->Save/Load disabled while playing (UE behavior)

Physics bodies/constraints/characters and script instances are
reconciled automatically by the existing per-frame sync on restore.

Engine/include/Engine/Serialization/SceneSerializer.h

@@ -55,6 +55,23 @@ namespace Engine::Serialization {
                                 Assets::AssetManager& assets,
                                 const std::string& inPath,
                                 std::string& outEnvironmentPath);
+
+        // ---- 内存快照（同一套实体读写，不同载体）----
+        // 消费者：编辑器 Play-In-Editor 快照恢复、Prefab、运行时动态
+        // 拼装场景。documentName 仅写入文档头 "Scene" 字段（注释性），
+        // 快照可传实体名或留空；environmentPath 语义同文件版。
+        static std::string SerializeToString(
+            const ECS::Scene& scene,
+            Assets::AssetManager& assets,
+            const std::string& documentName = {},
+            const std::string& environmentPath = {});
+
+        // 失败（YAML 异常/版本过高）返回 false 且不动现场。
+        // 实体悬空语义同文件版 Deserialize。
+        static bool DeserializeFromString(ECS::Scene& scene,
+                                          Assets::AssetManager& assets,
+                                          const std::string& yamlText,
+                                          std::string& outEnvironmentPath);
     };
 
 } // namespace Engine::Serialization