1.广角/主摄切换按钮已上移：

2.新增 isSwitchingLens 状态、避免切换镜头    stop 按钮禁用，不会触发 stopRecording
倍距按钮禁用，避免连续切换
ViewModel 层也兜底：isSwitchingLens=true 时直接拒绝 stop
3.合并失败兜底提示

lib/features/recording/pages/page_record.dart

@@ -173,7 +173,11 @@ class _RecordingPageState extends ConsumerState<RecordingPage> {
     if (!mounted) return;
     final latest = ref.read(recordingViewModelProvider).session;
     if (latest.fileSaveFailed) {
-      AppToast.show(latest.errorMessage ?? '保存到文件夹失败，请检查文件保存权限');
+      if (latest.segmentOutputPaths.isNotEmpty) {
+        AppToast.show('视频合并失败，已为你保存分段文件，可在相册中查看');
+      } else {
+        AppToast.show(latest.errorMessage ?? '保存到文件夹失败，请检查文件保存权限');
+      }
       return;
     }
     await _showRecordingSavedDialogIfNeeded();
@@ -374,6 +378,7 @@ class _RecordingHudLayer extends ConsumerWidget {
           m.session.notificationsGranted,
           m.session.isRecording,
           m.session.isStartingRecording,
+          m.session.isSwitchingLens,
           m.session.isTouchLocked,
           m.session.zoomRatio,
           m.session.minZoomRatio,
@@ -391,6 +396,7 @@ class _RecordingHudLayer extends ConsumerWidget {
       notificationsGranted,
       isRecording,
       isStartingRecording,
+      isSwitchingLens,
       isTouchLocked,
       zoomRatio,
       minZoomRatio,
@@ -408,6 +414,7 @@ class _RecordingHudLayer extends ConsumerWidget {
       notificationsGranted: notificationsGranted,
       isRecording: isRecording,
       isStartingRecording: isStartingRecording,
+      isSwitchingLens: isSwitchingLens,
       isTouchLocked: isTouchLocked,
       zoomRatio: zoomRatio,
       minZoomRatio: minZoomRatio,