Fix PIE deadlock when playing a scene without a primary camera

BuildFrame returned false (skip frame) when the game view had no
primary camera, skipping the WaitFence/ResetFence/submit chain - the
frame-slot fence never got signaled, so the next WaitFence on that
slot blocked forever. Reproduced instantly by playing an empty scene
with embedded PIE; latent in manual fullscreen preview too.

Game view now falls back to the editor camera instead of skipping the
frame, keeping the per-slot fence cadence intact. Viewport warning
text updated to mention the editor-view fallback.

App/editor/ViewportPanel.cpp

@@ -310,8 +310,8 @@ std::optional<entt::entity> ViewportPanel::Draw(const EditorContext &ctx,
             // 占一行布局 → 滚动条出现/消失循环 → avail 抖动 → 渲染分辨率
             // 每帧重报（视口疯狂顶动）；且默认字体无中文字形（显示 ?）
             if (playing) {
-                // 运行态无 primary 相机实体 = 游戏画面黑屏（BuildFrame
-                // 跳帧），提示补相机（Hierarchy 右键或 Add Component）
+                // 运行态无 primary 相机实体：渲染回退编辑器视角（保持帧
+                // 管线节拍），提示补相机
                 ECS::TransformComponent *camT = nullptr;
                 if (ViewCamera(ctx, camT) == nullptr) {
                     const ImVec2 imgMin = ImGui::GetItemRectMin();
@@ -319,8 +319,9 @@ std::optional<entt::entity> ViewportPanel::Draw(const EditorContext &ctx,
                             ImVec2(imgMin.x + 8.0f,
                                    imgMin.y + size.y * 0.5f),
                             IM_COL32(255, 120, 80, 230),
-                            "No primary camera entity - add a Camera "
-                            "component and check 'Primary'");
+                            "No primary camera entity - showing editor "
+                            "view. Add a Camera component and check "
+                            "'Primary'");
                 }
             }