fix(player): 使用雪碧图预热首帧兜底

test/playback/seek_progress_bar_test.dart

@@ -249,9 +249,7 @@ void main() {
     },
   );
 
-  testWidgets('should_show_frame_card_with_cover_then_hide_on_release', (
-    tester,
-  ) async {
+  testWidgets('should_show_frame_card_then_hide_on_release', (tester) async {
     final video = ValueNotifier<VideoPlayerValue>(
       const VideoPlayerValue(
         duration: Duration(seconds: 12),
@@ -272,7 +270,6 @@ void main() {
                 valueListenable: video,
                 semanticsLabel: 'Seek video',
                 episodeId: 'episode-1',
-                posterUrl: '',
                 thumbnailService: service,
                 onSeek: (_) async {},
               ),
@@ -289,7 +286,7 @@ void main() {
     await tester.pump();
 
     expect(find.byKey(const Key('scrub-preview-card')), findsOneWidget);
-    expect(find.byKey(const Key('scrub-preview-poster')), findsOneWidget);
+    expect(find.byKey(const Key('scrub-preview-background')), findsOneWidget);
     expect(find.byKey(const Key('scrub-preview-frame')), findsNothing);
     expect(find.textContaining('/ 00:12'), findsOneWidget);
     expect(

test/playback/sprite_thumbnail_axis_test.dart

@@ -114,6 +114,32 @@ void main() {
     expect(service.hasAxis('https://cdn.test/thumbnails.vtt'), isTrue);
   });
 
+  testWidgets('should_keep_zero_frames_ready_across_neighbor_episodes', (
+    tester,
+  ) async {
+    final loader = _FakeSpriteLoader(vtt: _spriteVtt);
+    final service = SpriteThumbnailAxisService(
+      loader: loader,
+      spriteCacheCapacity: 6,
+    );
+    addTearDown(service.dispose);
+    const current = 'https://cdn.test/episode-1/thumbnails.vtt';
+    const previous = 'https://cdn.test/episode-0/thumbnails.vtt';
+    const next = 'https://cdn.test/episode-2/thumbnails.vtt';
+
+    await Future.wait([
+      service.prepareAt(previous, Duration.zero),
+      service.prepareAt(current, Duration.zero),
+      service.prepareAt(next, Duration.zero),
+    ]);
+
+    for (final url in [previous, current, next]) {
+      expect(service.hasAxis(url), isTrue);
+      expect(service.cachedFrameAt(url, Duration.zero), isNotNull);
+    }
+    expect(loader.spriteCalls, hasLength(6));
+  });
+
   testWidgets('should_refresh_vtt_after_ten_minute_ttl', (tester) async {
     var now = DateTime(2026, 10, 22);
     final loader = _FakeSpriteLoader(vtt: _spriteVtt);