refactor(player): clarify requested vs session-effective media source id

lib/screens/video_player/parts/episode_navigation.dart

@@ -136,12 +136,12 @@ extension _VideoPlayerEpisodeNavigationMethods on VideoPlayerScreenState {
     final currentSubtitleStreamId = _selectedSourceSubtitleStreamIdForControls(_sourceSubtitleTracksForControls());
     final effectiveSubtitleStreamId = newSubtitleStreamId ?? currentSubtitleStreamId;
     final effectiveMediaSourceId = newMediaIndex != null
-        ? PlaybackSession.mediaSourceIdForIndex(_availableVersions, effectiveMediaIndex) ?? _selectedMediaSourceId
-        : _selectedMediaSourceId;
+        ? PlaybackSession.mediaSourceIdForIndex(_availableVersions, effectiveMediaIndex) ?? _requestedMediaSourceId
+        : _requestedMediaSourceId;
 
     final isVersionChange =
         effectiveMediaIndex != _effectiveSelectedMediaIndex ||
-        (_selectedMediaSourceId != null && effectiveMediaSourceId != _selectedMediaSourceId);
+        (_requestedMediaSourceId != null && effectiveMediaSourceId != _requestedMediaSourceId);
     final isPresetChange = effectivePreset != _selectedQualityPreset;
     final isAudioChange = effectiveAudioStreamId != _selectedAudioStreamId;
     final isSubtitleChange = newSubtitleStreamId != null && effectiveSubtitleStreamId != currentSubtitleStreamId;

lib/screens/video_player/parts/playback_start.dart

@@ -138,7 +138,7 @@ extension _VideoPlayerPlaybackStartMethods on VideoPlayerScreenState {
         playbackContext = await playbackResolver.resolve(
           metadata: _currentMetadata,
           selectedMediaIndex: _effectiveSelectedMediaIndex,
-          selectedMediaSourceId: _selectedMediaSourceId,
+          selectedMediaSourceId: _requestedMediaSourceId,
           offlineLibraryMode: true,
           qualityPreset: _selectedQualityPreset,
           selectedAudioStreamId: _selectedAudioStreamId,
@@ -174,7 +174,7 @@ extension _VideoPlayerPlaybackStartMethods on VideoPlayerScreenState {
         PlaybackSession.fromContext(
           playbackContext,
           requestedQualityPreset: _selectedQualityPreset,
-          requestedMediaSourceId: _selectedMediaSourceId,
+          requestedMediaSourceId: _requestedMediaSourceId,
         ),
       );
 

lib/screens/video_player/parts/seeking.dart

@@ -65,7 +65,7 @@ extension _VideoPlayerSeekingMethods on VideoPlayerScreenState {
       final playbackContext = await playbackResolver.resolve(
         metadata: replacementMetadata,
         selectedMediaIndex: _effectiveSelectedMediaIndex,
-        selectedMediaSourceId: _selectedMediaSourceId,
+        selectedMediaSourceId: _requestedMediaSourceId,
         offlineLibraryMode: false,
         qualityPreset: _selectedQualityPreset,
         selectedAudioStreamId: _selectedAudioStreamId,
@@ -84,7 +84,7 @@ extension _VideoPlayerSeekingMethods on VideoPlayerScreenState {
       final session = PlaybackSession.fromContext(
         playbackContext,
         requestedQualityPreset: _selectedQualityPreset,
-        requestedMediaSourceId: _selectedMediaSourceId,
+        requestedMediaSourceId: _requestedMediaSourceId,
       );
 
       final attachesSubsAtOpen = currentPlayer.attachesExternalSubtitlesAtOpen;

lib/screens/video_player_screen.dart

@@ -270,7 +270,12 @@ class VideoPlayerScreenState extends State<VideoPlayerScreen> with WidgetsBindin
   bool _showPlayNextDialog = false;
   bool _isPhone = false;
   late int _effectiveSelectedMediaIndex;
-  String? _selectedMediaSourceId;
+
+  /// Media source id to request on the next resolve: the caller's initial
+  /// selection, then re-synced to the session's post-fallback effective id
+  /// by [_commitPlaybackSession]. Post-resolve consumers must read
+  /// `_playbackSession.mediaSourceId`, never this field.
+  String? _requestedMediaSourceId;
   bool get _offlineLibraryMode => widget.isOffline;
 
   // Transcode / quality state
@@ -454,7 +459,7 @@ class VideoPlayerScreenState extends State<VideoPlayerScreen> with WidgetsBindin
   void _commitPlaybackSession(PlaybackSession session) {
     _playbackSession = session;
     _effectiveSelectedMediaIndex = session.mediaIndex;
-    _selectedMediaSourceId = session.mediaSourceId;
+    _requestedMediaSourceId = session.mediaSourceId;
     _selectedQualityPreset = session.qualityPreset;
     _selectedAudioStreamId = session.audioStreamId;
   }
@@ -490,7 +495,7 @@ class VideoPlayerScreenState extends State<VideoPlayerScreen> with WidgetsBindin
     _activeId = widget.metadata.id;
     _activeMediaIndex = widget.selectedMediaIndex;
     _effectiveSelectedMediaIndex = widget.selectedMediaIndex;
-    _selectedMediaSourceId = widget.selectedMediaSourceId;
+    _requestedMediaSourceId = widget.selectedMediaSourceId;
 
     // Reused across in-place quality/version/audio switches so the
     // server-side transcode session is preserved.
@@ -683,7 +688,7 @@ class VideoPlayerScreenState extends State<VideoPlayerScreen> with WidgetsBindin
         _playbackDataFuture = playbackResolver.resolve(
           metadata: _currentMetadata,
           selectedMediaIndex: _effectiveSelectedMediaIndex,
-          selectedMediaSourceId: _selectedMediaSourceId,
+          selectedMediaSourceId: _requestedMediaSourceId,
           offlineLibraryMode: false,
           qualityPreset: _selectedQualityPreset,
           selectedAudioStreamId: _selectedAudioStreamId,