perf(player): 限制雪碧图与海报预缓存内存并给播放位置缓存加容量上限

雪碧图解码从 2048 降到 1024、缓存从 8 张降到 3 张，避免擦洗预览驻留十几 MB 位图；
相邻集海报预缓存改用 ResizeImage 按 360/720 宽解码，不再缓存整张原图；
播放位置缓存按写入时间淘汰，最多保留 100 条，长会话不再只增不减；
观看记录详情失败日志加 kDebugMode 保护，release 下不再逐条打堆栈。

test/playback/playback_position_cache_test.dart

@@ -89,6 +89,25 @@ void main() {
     expect(cache.latestForDrama('drama-a')?.episodeId, 'episode-2');
   });
 
+  test('should_evict_oldest_entry_when_capacity_exceeded', () {
+    var now = DateTime(2026, 10, 22, 12);
+    final cache = PlaybackPositionCache(now: () => now, maxEntries: 2);
+
+    for (final episodeId in ['episode-1', 'episode-2', 'episode-3']) {
+      cache.save(
+        episodeId: episodeId,
+        dramaId: 'drama-a',
+        position: const Duration(seconds: 30),
+        duration: const Duration(minutes: 3),
+      );
+      now = now.add(const Duration(minutes: 1));
+    }
+
+    expect(cache.read('episode-1', dramaId: 'drama-a'), isNull);
+    expect(cache.read('episode-2', dramaId: 'drama-a'), isNotNull);
+    expect(cache.read('episode-3', dramaId: 'drama-a'), isNotNull);
+  });
+
   test('should_reject_expired_cached_position', () {
     var now = DateTime(2026, 10, 22, 12);
     final cache = PlaybackPositionCache(now: () => now);