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/main.cpp

@@ -65,8 +65,11 @@ namespace
         {
             // 编辑器形态：驱动编辑器相机（RMB 飞行 / MMB 焦点环绕 /
             // 滚轮推拉）。暂停（timeScale=0）时仍可飞行——unscaled dt。
+            // PIE 内嵌播放：渲染源已切 primary 相机，编辑器相机停驱
+            //（否则与游戏脚本抢 WASD，Stop 后视角也被偷跑）。
             // 游戏形态：相机实体由游戏逻辑驱动（本沙盒无逻辑 = 静止）
-            if (GetRenderer().GetDisplayMode() == kDisplayViewport)
+            if (GetRenderer().GetDisplayMode() == kDisplayViewport &&
+                !m_Editor->IsPlaying())
             {
                 GetRenderer().GetEditorCamera().Update(GetWindow(),
                                                        frameTime.UnscaledDt());