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.

App/editor/Editor.h

@@ -55,6 +55,17 @@ public:
     // 请求退出（弹确认框；窗口关闭事件走这里）
     void RequestQuit() { m_ShowQuitConfirm = true; }
 
+    // ---- PIE（Play-In-Editor，UE 工具栏语义）----
+    bool IsPlaying() const { return m_Playing; }
+    // Play：内存快照当前场景 → 切游戏渲染（primary 相机）→ 解冻时间；
+    // Stop：快照原子换血恢复 → 回编辑器渲染 → 冻结时间。
+    // 编辑器态默认 timeScale=0（物理冻结 = UE 语义），Play 解冻
+    void TogglePlay();
+
+private:
+    void DoPlay();    // TogglePlay 的 Play 半程（快照 + 切渲染 + 解冻）
+    void DoStop();    // TogglePlay 的 Stop 半程（恢复 + 回编辑器 + 冻结）
+
     // ---- 选中状态 ----
     // 同步描边：Renderer 落帧后 SelectionMask/Outline 只画选中实体
     //（bits = 实体位模式，0 = 清选中）
@@ -109,6 +120,10 @@ private:
     bool m_ShowSaveModal = false;
     bool m_ShowLoadModal = false;
     char m_ScenePathBuf[256] = "scenes/sandbox.scene";
+
+    // PIE 状态：Play 时场景 YAML 内存快照，Stop 时原子换血恢复
+    bool m_Playing = false;
+    std::string m_PlaySnapshot;
 };
 
 #endif //HELLOVULKAN_EDITOR_H