fix(player): 修复详情往返播放位置丢失

test/playback/playback_position_cache_test.dart

@@ -0,0 +1,69 @@
+// Author: fruit.com.cn
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:fruitshorts/core/data/services/playback_position_cache.dart';
+
+void main() {
+  test('should_store_and_read_recent_position_when_drama_matches', () {
+    final now = DateTime(2026, 10, 22, 12);
+    final cache = PlaybackPositionCache(now: () => now);
+
+    cache.save(
+      episodeId: 'episode-3',
+      dramaId: 'drama-a',
+      position: const Duration(minutes: 1, seconds: 20),
+      duration: const Duration(minutes: 3),
+    );
+
+    final entry = cache.read('episode-3', dramaId: 'drama-a');
+    expect(entry, isNotNull);
+    expect(entry!.position, const Duration(minutes: 1, seconds: 20));
+    expect(entry.duration, const Duration(minutes: 3));
+    expect(entry.updatedAt, now);
+  });
+
+  test('should_reject_cached_position_when_drama_does_not_match', () {
+    final cache = PlaybackPositionCache();
+    cache.save(
+      episodeId: 'episode-3',
+      dramaId: 'drama-a',
+      position: const Duration(seconds: 80),
+      duration: const Duration(minutes: 3),
+    );
+
+    expect(cache.read('episode-3', dramaId: 'drama-b'), isNull);
+  });
+
+  test('should_use_latest_saved_episode_when_timestamps_match', () {
+    final now = DateTime(2026, 10, 22, 12);
+    final cache = PlaybackPositionCache(now: () => now);
+    cache.save(
+      episodeId: 'episode-1',
+      dramaId: 'drama-a',
+      position: const Duration(seconds: 30),
+      duration: const Duration(minutes: 3),
+    );
+    cache.save(
+      episodeId: 'episode-2',
+      dramaId: 'drama-a',
+      position: const Duration(seconds: 45),
+      duration: const Duration(minutes: 3),
+    );
+
+    expect(cache.latestForDrama('drama-a')?.episodeId, 'episode-2');
+  });
+
+  test('should_reject_expired_cached_position', () {
+    var now = DateTime(2026, 10, 22, 12);
+    final cache = PlaybackPositionCache(now: () => now);
+    cache.save(
+      episodeId: 'episode-3',
+      dramaId: 'drama-a',
+      position: const Duration(seconds: 80),
+      duration: const Duration(minutes: 3),
+    );
+    now = now.add(const Duration(days: 1));
+
+    expect(cache.read('episode-3', dramaId: 'drama-a'), isNull);
+  });
+}