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/EditorContext.h

@@ -42,6 +42,10 @@ struct EditorContext {
     // 无关；面板经此做 F 焦点/Gizmo 视图/对齐跳转）
     Engine::EditorCamera *editorCamera = nullptr;
 
+    // PIE 运行态（Play-In-Editor）：Editor 持有，只读指针。
+    // 面板门控用——运行态禁 Gizmo/编辑器热键，渲染源已是 primary 相机
+    const bool *playState = nullptr;
+
     std::function<int()> getEnvIndex; // 当前环境表下标
     std::function<void(int)> setEnvIndex;
 };