fix(player): 雪碧图缓存补齐槽位预取量，清屏按钮贴近进度条

三个渲染槽位各预取 2 张雪碧图，容量 3 装不下会导致「重建→重新下载解码→
notify→重建」的死循环，真机持续发烫掉帧；容量恢复为 8 并只在未命中时预取。
首页清屏态的退出按钮由距底 56 收到 24，贴着进度条上方。

lib/pages/player/player_view.dart

@@ -189,9 +189,10 @@ class _PlayerPageState extends State<PlayerPage>
     _thumbnailService = PlaybackThumbnailService(
       sourceResolver: _resolveThumbnailSource,
     );
-    // 雪碧图每张解码后是整块位图，保留 3 部剧足够擦洗回退，8 部在低端机
-    // 容易把图片缓存顶满甚至触发 OOM。
-    _spriteAxisService = SpriteThumbnailAxisService(spriteCacheCapacity: 3)
+    // 容量必须盖住「同时预取的数量」：渲染栈有上一集/当前集/下一集三个槽位，
+    // 每槽预取当前 cue + 1 个相邻 cue，约 6 张。容量小于这个数时，每次重建都会
+    // 把别处刚解码的雪碧图挤掉，形成「重建→重新下载解码→notify→重建」的死循环。
+    _spriteAxisService = SpriteThumbnailAxisService(spriteCacheCapacity: 8)
       ..addListener(_handleSpriteFrameChanged);
     WidgetsBinding.instance.addObserver(this);
     logic.start(
@@ -1013,19 +1014,19 @@ class _PlayerPageState extends State<PlayerPage>
     final thumbnailReady =
         thumbnailsUrl != null &&
         _spriteAxisService.cachedFrameAt(thumbnailsUrl, Duration.zero) != null;
-    if (!locked && !posterReady && thumbnailsUrl != null) {
+    // 只在没命中时预取：prepareAt 会触发下载解码并 notifyListeners -> 重建，
+    // 无条件调用会让每次重建都排一次异步任务。
+    if (!locked && !posterReady && !thumbnailReady && thumbnailsUrl != null) {
       unawaited(_spriteAxisService.prepareAt(thumbnailsUrl, Duration.zero));
     }
     if (selected &&
         !locked &&
         videoPort != null &&
         videoPort.controller.value.isInitialized) {
-      unawaited(
-        _spriteAxisService.prepareAt(
-          thumbnailsUrl,
-          videoPort.controller.value.position,
-        ),
-      );
+      final position = videoPort.controller.value.position;
+      if (_spriteAxisService.cachedFrameAt(thumbnailsUrl, position) == null) {
+        unawaited(_spriteAxisService.prepareAt(thumbnailsUrl, position));
+      }
     }
     final pagePosition = indices.indexOf(index);
     return PlayerRenderSlotData(
@@ -1286,7 +1287,8 @@ class _PlayerPageState extends State<PlayerPage>
         if (widget.homeMode && !_chromeVisible && !_boosting && !_scrubbing)
           Positioned(
             right: 12,
-            bottom: 56,
+            // 进度条触控带 16pt + 8pt 间距，退出清屏按钮贴着进度条上方。
+            bottom: 24,
             child: ExitClearScreenButton(
               key: const Key('home-feed-exit-clear-screen'),
               tooltip: l10n.exitClearScreen,

test/playback/sprite_thumbnail_axis_test.dart

@@ -159,6 +159,31 @@ void main() {
     expect(loader.spriteCalls, hasLength(6));
   });
 
+  testWidgets('should_not_reload_three_slots_sprites_when_capacity_fits', (
+    tester,
+  ) async {
+    final loader = _FakeSpriteLoader(vtt: _spriteVtt);
+    // 播放页三个槽位各预取 2 张，容量必须 >= 6，否则每轮重建都会重新下载解码。
+    final service = SpriteThumbnailAxisService(
+      loader: loader,
+      spriteCacheCapacity: 8,
+    );
+    addTearDown(service.dispose);
+    const urls = [
+      'https://cdn.test/episode-0/thumbnails.vtt',
+      'https://cdn.test/episode-1/thumbnails.vtt',
+      'https://cdn.test/episode-2/thumbnails.vtt',
+    ];
+
+    for (var round = 0; round < 3; round++) {
+      for (final url in urls) {
+        await service.prepareAt(url, Duration.zero);
+      }
+    }
+
+    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);