fix(player): 使用高清首帧并稳定切集占位

test/player/player_opening_frame_test.dart

@@ -17,6 +17,53 @@ sprite_001.jpg#xywh=0,0,160,90
 ''';
 
 void main() {
+  test('should_choose_poster_before_thumbnails_when_both_are_available', () {
+    final service = SpriteThumbnailAxisService(loader: _FakeSpriteLoader());
+    addTearDown(service.dispose);
+
+    final frame =
+        playerOpeningFrame(
+              'https://cdn.test/thumbnails.vtt',
+              posterUrl: 'https://cdn.test/poster.jpg',
+              spriteSource: service,
+              fit: BoxFit.cover,
+            )
+            as PlayerOpeningFrame;
+
+    expect(frame.posterUrl, 'https://cdn.test/poster.jpg');
+    expect(frame.thumbnailsUrl, 'https://cdn.test/thumbnails.vtt');
+    expect(frame.fit, BoxFit.cover);
+    expect(
+      playerOpeningFallback(
+        posterUrl: frame.posterUrl,
+        thumbnailsUrl: frame.thumbnailsUrl,
+      ),
+      PlayerOpeningFallback.poster,
+    );
+  });
+
+  testWidgets('should_use_poster_without_decoding_sprite_when_poster_exists', (
+    tester,
+  ) async {
+    final loader = _FakeSpriteLoader();
+    final service = SpriteThumbnailAxisService(loader: loader);
+    addTearDown(service.dispose);
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: PlayerOpeningFrame(
+          posterUrl: 'https://cdn.test/poster.jpg',
+          thumbnailsUrl: 'https://cdn.test/thumbnails.vtt',
+          spriteSource: service,
+        ),
+      ),
+    );
+
+    expect(find.byKey(const Key('player-opening-poster')), findsOneWidget);
+    expect(find.byKey(const Key('player-opening-sprite')), findsNothing);
+    expect(loader.vttCalls, 0);
+  });
+
   testWidgets('should_return_no_opening_frame_when_thumbnails_is_empty', (
     tester,
   ) async {