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.

Engine/src/Renderer/SceneRenderer.cpp

@@ -278,16 +278,21 @@ namespace Engine
         if (m_DisplayMode == kDisplayViewport && !m_GameView) {
             camTransform = &m_EditorCamera.GetTransform();
             camera = &m_EditorCamera.GetController();
-        } else if (auto* cameraComp = scene.GetPrimaryCamera(&camTransform)) {
-            camera = &cameraComp->camera;
-        }
-        if (!camera || !camTransform) {
-            // 游戏形态无 primary 相机：本帧跳过（安全网）
-            return false;   // 拾取请求未消费，留待下一帧重试
+        } else {
+            if (auto* cameraComp = scene.GetPrimaryCamera(&camTransform)) {
+                camera = &cameraComp->camera;
+            }
+            if (!camera || !camTransform) {
+                // 游戏形态无 primary 相机：回退编辑器相机渲染（视口有
+                // 橙色提示引导补相机）。绝不能跳帧——跳过 submit 会漏
+                // 掉本槽位 fence 的 signal，下一轮该槽位 WaitFence 永
+                // 久阻塞 = 整个渲染死锁（空场景 Play 卡死的根因）
+                camTransform = &m_EditorCamera.GetTransform();
+                camera = &m_EditorCamera.GetController();
+            }
         }
 
-        // 拾取请求落帧（一次性消费：本帧 HitProxy 渲染 + 像素拷贝）。
-        // 放在相机校验之后：跳帧时请求保留，不丢点击
+        // 拾取请求落帧（一次性消费：本帧 HitProxy 渲染 + 像素拷贝）
         m_Frame.pickRequest = m_PendingPick;
         m_PendingPick.active = false;
         // 选中实体落帧（持续状态，不做一次性清除）。