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.

Engine/include/Engine/Renderer/Renderer.h

@@ -142,6 +142,7 @@ namespace Engine {
         // ---- 显示模式（Editor 视口 ↔ 全屏直出；帧首同步生效）----
         int GetDisplayMode() const { return m_DesiredDisplayMode; }
         void SetDisplayMode(int mode);   // cpp 内定义（SceneRenderer 前置声明）
+        void SetGameView(bool gameView);   // PIE 内嵌播放（cpp 转发 SceneRenderer）
         // 编辑器相机（Scene view 恒用，与场景相机实体无关）。转发
         // SceneRenderer 持有的实例：Application 驱动 Update，面板读写位姿
         EditorCamera& GetEditorCamera();   // cpp 内定义（SceneRenderer 完整类型）

Engine/include/Engine/Renderer/SceneRenderer.h

@@ -66,6 +66,10 @@ namespace Engine {
         void SetLodEnabled(bool b) { m_LodEnabled = b; }
         void SetDebugView(int v) { m_DebugView = v; }
         void SetDisplayMode(int m) { m_DisplayMode = m; }   // kDisplayDirect / kDisplayViewport
+        // PIE 内嵌播放（UE Play-In-Editor 语义）：viewport 模式下也用
+        // primary 相机渲染进 sceneTex（Play 后 Scene view 显示游戏画面，
+        // 编辑器 UI 保持可用）。kDisplayDirect 恒为游戏形态不受此影响
+        void SetGameView(bool g) { m_GameView = g; }
         // 编辑器相机（视口 UI 状态，SceneRenderer 持有 = 渲染源在此分派）；
         // Application 驱动 Update，ViewportPanel/Inspector 经 Renderer 门面访问
         EditorCamera& GetEditorCamera() { return m_EditorCamera; }
@@ -171,6 +175,7 @@ namespace Engine {
         bool m_LodEnabled = true;   // 关 = 全部强制 LOD0（选级跳过）
         int m_DebugView = 0;
         int m_DisplayMode = kDisplayViewport;   // Editor 视口为默认形态
+        bool m_GameView = false;   // PIE 内嵌播放（Editor Play/Stop 驱动）
     };
 
 } // namespace Engine