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/ViewportPanel.cpp

@@ -69,12 +69,14 @@ namespace
 
     // 视图相机分派（必须与 SceneRenderer::BuildFrame 渲染源同一规则，
     // 否则 Gizmo 杆与画面错位）：编辑器形态 = EditorCamera（Scene view
-    // 专用，与相机实体无关）；游戏形态 = primary 相机实体。
+    // 专用，与相机实体无关）；游戏形态（kDisplayDirect 全屏预览或 PIE
+    // 内嵌播放）= primary 相机实体。
     // 返回 nullptr = 游戏形态且无 primary 相机（本帧无 Gizmo）
     Engine::Camera* ViewCamera(const EditorContext& ctx,
                                ECS::TransformComponent*& outT)
     {
-        if (ctx.renderer->GetDisplayMode() == kDisplayViewport)
+        const bool playing = ctx.playState && *ctx.playState;
+        if (!playing && ctx.renderer->GetDisplayMode() == kDisplayViewport)
         {
             outT = &ctx.editorCamera->GetTransform();
             return &ctx.editorCamera->GetController();
@@ -281,12 +283,14 @@ std::optional<entt::entity> ViewportPanel::Draw(const EditorContext &ctx,
             }
 
             // Gizmo 状态提示（左上角 drawlist 叠加，原因同下：不占布局行）
-            // PIE 运行态 = 黄色 PLAYING 提示替代编辑状态
+            // PIE 运行态 = 黄色 PLAYING/PAUSED 提示替代编辑状态
             {
                 char tip[64];
                 if (playing) {
+                    const bool paused = ctx.pausedState && *ctx.pausedState;
                     std::snprintf(tip, sizeof(tip),
-                                  "PLAYING - Ctrl+P to stop");
+                                  paused ? "PAUSED - Ctrl+P to stop"
+                                         : "PLAYING - Ctrl+P to stop");
                 } else {
                     std::snprintf(tip, sizeof(tip),
                                   "Space: %s [T]   Snap: Ctrl",
@@ -335,8 +339,9 @@ std::optional<entt::entity> ViewportPanel::Draw(const EditorContext &ctx,
 
             // 点击拾取：单击 Image 且未命中 UI 遮挡（IsItemHovered 已含）。
             // 抓杆时不出 pick（IsOver 悬停杆/IsUsing 拖拽中）——否则按在
-            // 操纵杆上的点击会穿透发拾取请求
-            if (ImGui::IsItemHovered() && ImGui::IsMouseClicked(0) &&
+            // 操纵杆上的点击会穿透发拾取请求。PIE 运行态不出拾取：
+            // 点击 = 游戏输入（UE Play 语义，编辑操作冻结）
+            if (!playing && ImGui::IsItemHovered() && ImGui::IsMouseClicked(0) &&
                 !ImGuizmo::IsOver() && !ImGuizmo::IsUsing()) {
                 RequestPickAt(ctx, size);
             }