fix(player): 记住拖动后的实际播放位置

test/playback/playback_position_cache_test.dart

@@ -4,6 +4,42 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:fruitshorts/core/data/services/playback_position_cache.dart';
 
 void main() {
+  test('should_clamp_invalid_or_out_of_range_position_when_caching', () {
+    expect(
+      clampPlaybackPosition(
+        const Duration(seconds: -1),
+        const Duration(seconds: 10),
+      ),
+      Duration.zero,
+    );
+    expect(
+      clampPlaybackPosition(
+        const Duration(seconds: 12),
+        const Duration(seconds: 10),
+      ),
+      const Duration(seconds: 10),
+    );
+    expect(
+      clampPlaybackPosition(
+        const Duration(seconds: 5),
+        const Duration(seconds: 10),
+      ),
+      const Duration(seconds: 5),
+    );
+
+    final cache = PlaybackPositionCache();
+    cache.save(
+      episodeId: 'episode-3',
+      dramaId: 'drama-a',
+      position: const Duration(minutes: 4),
+      duration: const Duration(minutes: 3),
+    );
+    expect(
+      cache.read('episode-3', dramaId: 'drama-a')?.position,
+      const Duration(minutes: 3),
+    );
+  });
+
   test('should_store_and_read_recent_position_when_drama_matches', () {
     final now = DateTime(2026, 10, 22, 12);
     final cache = PlaybackPositionCache(now: () => now);