fix(player): 修复详情往返播放位置丢失

lib/core/data/services/playback_position_cache.dart

@@ -0,0 +1,79 @@
+// Author: fruit.com.cn
+
+const Duration _defaultMaxAge = Duration(hours: 12);
+
+/// 进程内最近播放位置。
+class CachedPlaybackPosition {
+  const CachedPlaybackPosition({
+    required this.episodeId,
+    required this.dramaId,
+    required this.position,
+    required this.duration,
+    required this.updatedAt,
+  });
+
+  final String episodeId;
+  final String dramaId;
+  final Duration position;
+  final Duration duration;
+  final DateTime updatedAt;
+}
+
+/// 按集缓存最近播放位置，仅当前进程有效。
+class PlaybackPositionCache {
+  PlaybackPositionCache({
+    DateTime Function()? now,
+    this.maxAge = _defaultMaxAge,
+  }) : _now = now ?? DateTime.now;
+
+  final DateTime Function() _now;
+  final Duration maxAge;
+  final Map<String, CachedPlaybackPosition> _positions = {};
+
+  /// 保存最近有效播放位置；零位置不会覆盖已有记录。
+  void save({
+    required String episodeId,
+    required String dramaId,
+    required Duration position,
+    required Duration duration,
+  }) {
+    if (episodeId.isEmpty || dramaId.isEmpty || position <= Duration.zero) {
+      return;
+    }
+    _positions[episodeId] = CachedPlaybackPosition(
+      episodeId: episodeId,
+      dramaId: dramaId,
+      position: position,
+      duration: duration,
+      updatedAt: _now(),
+    );
+  }
+
+  /// 读取同剧同集且未过期的最近位置。
+  CachedPlaybackPosition? read(String episodeId, {required String dramaId}) {
+    final entry = _positions[episodeId];
+    if (!_isValid(entry, dramaId)) return null;
+    return entry;
+  }
+
+  /// 读取指定短剧最近更新的有效位置。
+  CachedPlaybackPosition? latestForDrama(String dramaId) {
+    CachedPlaybackPosition? latest;
+    for (final entry in _positions.values) {
+      if (!_isValid(entry, dramaId)) continue;
+      if (latest == null || !entry.updatedAt.isBefore(latest.updatedAt)) {
+        latest = entry;
+      }
+    }
+    return latest;
+  }
+
+  bool _isValid(CachedPlaybackPosition? entry, String dramaId) {
+    if (entry == null ||
+        entry.dramaId != dramaId ||
+        entry.position <= Duration.zero) {
+      return false;
+    }
+    return _now().difference(entry.updatedAt) <= maxAge;
+  }
+}