fix(player): 收紧图片与快速切集资源占用

lib/core/data/services/playback_session.dart

@@ -81,6 +81,8 @@ class PlaybackSession extends ChangeNotifier {
   // Includes initialization and asynchronous disposal, until native release ends.
   int get livePlayerCount => _live.length;
   int? get preloadedIndex => _next?.prepared == true ? _next?.index : null;
+  int? get previousPreloadedIndex =>
+      _previous?.prepared == true ? _previous?.index : null;
   Future<void> get playbackSettled => _operations;
   Future<void> get preloadSettled => _preloadTask;
 

lib/core/data/services/sprite_thumbnail_axis.dart

@@ -9,7 +9,7 @@ import '../../config/app_config.dart';
 import 'sprite_thumbnail_asset_loader.dart';
 
 const Duration spriteVttCacheTtl = Duration(minutes: 10);
-const int spriteImageCacheCapacity = 2;
+const int spriteImageCacheCapacity = 1;
 
 /// 单个 VTT cue 对应的 sprite 裁剪区域。
 class SpriteThumbnailCue {
@@ -148,7 +148,6 @@ class SpriteThumbnailAxisService extends ChangeNotifier
   final Set<String> _failedSprites = {};
   bool _disposed = false;
 
-  @visibleForTesting
   int get spriteCacheSize => _spriteCache.length;
 
   @visibleForTesting
@@ -156,7 +155,7 @@ class SpriteThumbnailAxisService extends ChangeNotifier
       .map((url) => Uri.tryParse(url)?.pathSegments.lastOrNull ?? url)
       .toList();
 
-  /// 播放器就绪时预取当前 cue 与相邻 sprite，拖动帧不触发网络或解码。
+  /// 播放器就绪时预取当前 cue；容量大于 1 时顺带预取相邻 sprite。
   @override
   Future<void> prepareAt(String? thumbnailsUrl, Duration position) async {
     final url = _normalizeUrl(thumbnailsUrl);
@@ -269,6 +268,7 @@ class SpriteThumbnailAxisService extends ChangeNotifier
     if (current == null) return const [];
     final index = axis.cues.indexOf(current);
     final candidates = <SpriteThumbnailCue>[current];
+    if (spriteCacheCapacity <= 1) return candidates;
     for (final direction in const [1, -1]) {
       for (var step = 1; step < axis.cues.length; step++) {
         final neighborIndex = index + direction * step;