fix(player): 收紧图片与快速切集资源占用

test/playback/sprite_thumbnail_axis_test.dart

@@ -75,7 +75,9 @@ void main() {
     expect(axis.cues, isEmpty);
   });
 
-  testWidgets('should_keep_only_two_decoded_sprites_in_lru', (tester) async {
+  testWidgets('should_keep_one_sprite_and_reload_after_eviction', (
+    tester,
+  ) async {
     final loader = _FakeSpriteLoader(vtt: _spriteVtt);
     final service = SpriteThumbnailAxisService(
       loader: loader,
@@ -84,20 +86,37 @@ void main() {
     addTearDown(service.dispose);
 
     await service.prepareAt('https://cdn.test/thumbnails.vtt', Duration.zero);
-    expect(service.spriteCacheSize, 2);
-    expect(
-      service.spriteCacheKeys,
-      containsAll(['sprite_001.jpg', 'sprite_002.jpg']),
-    );
-
     await service.prepareAt(
       'https://cdn.test/thumbnails.vtt',
       const Duration(milliseconds: 2500),
     );
+    await service.prepareAt(
+      'https://cdn.test/thumbnails.vtt',
+      const Duration(milliseconds: 3500),
+    );
 
-    expect(service.spriteCacheSize, 2);
-    expect(service.spriteCacheKeys, contains('sprite_002.jpg'));
-    expect(service.spriteCacheKeys, isNot(contains('sprite_001.jpg')));
+    expect(service.spriteCacheSize, 1);
+    expect(service.spriteCacheKeys, ['sprite_003.jpg']);
+    expect(loader.images[0].debugDisposed, isTrue);
+    expect(loader.images[1].debugDisposed, isTrue);
+    expect(loader.images[2].debugDisposed, isFalse);
+    expect(
+      service.cachedFrameAt(
+        'https://cdn.test/thumbnails.vtt',
+        const Duration(milliseconds: 3500),
+      ),
+      isNotNull,
+    );
+
+    await service.prepareAt('https://cdn.test/thumbnails.vtt', Duration.zero);
+    expect(service.spriteCacheSize, 1);
+    expect(service.spriteCacheKeys, ['sprite_001.jpg']);
+    expect(loader.images[2].debugDisposed, isTrue);
+    expect(loader.images[3].debugDisposed, isFalse);
+    expect(
+      service.cachedFrameAt('https://cdn.test/thumbnails.vtt', Duration.zero),
+      isNotNull,
+    );
   });
 
   testWidgets('should_cache_vtt_and_retry_once_after_403', (tester) async {
@@ -229,6 +248,7 @@ class _FakeSpriteLoader implements SpriteThumbnailAssetLoader {
   final List<Object>? vttResponses;
   int vttCalls = 0;
   final List<String> spriteCalls = [];
+  final List<ui.Image> images = [];
 
   @override
   Future<String?> loadVtt(Uri uri) async {
@@ -244,6 +264,7 @@ class _FakeSpriteLoader implements SpriteThumbnailAssetLoader {
   Future<SpriteThumbnailDecodedImage?> loadSprite(Uri uri) async {
     spriteCalls.add(uri.pathSegments.last);
     final image = await _createImage(320, 180);
+    images.add(image);
     return SpriteThumbnailDecodedImage(
       image: image,
       originalWidth: 320,