PIE: viewport-embedded playback with UE-style main toolbar

Play no longer switches to fullscreen direct rendering: PIE now keeps
the editor UI alive and renders the primary camera into the Viewport
panel (new SceneRenderer::SetGameView flag; kDisplayDirect stays a
manual fullscreen preview for later release form). Editor camera stops
being driven during PIE, and viewport click-pick is gated off while
playing (clicks are game input now).

The Play/Stop buttons move out of the menu bar into a dedicated main
toolbar row below it (centered, UE Main Toolbar semantics): green Play
when idle; Pause (yellow, highlighted while paused) + Stop (orange)
while playing. Ctrl+P hotkey kept. Stats "Pause Simulation" merged
into the toolbar Pause; viewport overlay shows PAUSED accordingly.

App/editor/EditorContext.h

@@ -29,6 +29,8 @@ struct EditorContext {
     // 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;
+    // PIE 暂停态（timeScale=0）：Editor 持有，只读指针（视口 PAUSED 提示）
+    const bool *pausedState = nullptr;
 };
 
 #endif //HELLOVULKAN_EDITORCONTEXT_H