fix(player): 使用雪碧图预热首帧兜底

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);