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。

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);