fix(player): 重构单层过渡并阻止锁定集串播

test/playback/playback_session_test.dart

@@ -32,6 +32,92 @@ void main() {
     await session.close();
   });
 
+  test('should_release_previous_player_immediately_when_selecting_locked', () async {
+    final lockedEpisodes = [
+      Episode(
+        id: 'free',
+        number: 1,
+        title: 'Free',
+        isFree: true,
+        videoUrl: 'https://example.test/free.mp4',
+      ),
+      Episode(
+        id: 'locked',
+        number: 2,
+        title: 'Locked',
+        isFree: false,
+        locked: true,
+        videoUrl: 'https://example.test/should-not-play.mp4',
+      ),
+      Episode(
+        id: 'next',
+        number: 3,
+        title: 'Next',
+        isFree: true,
+        videoUrl: 'https://example.test/next.mp4',
+      ),
+    ];
+    final factory = FakeFactory();
+    final session = PlaybackSession(
+      episodes: lockedEpisodes,
+      factory: factory.create,
+      canPlay: (episode) => !episode.locked,
+    );
+    await session.select(0, preloadIndex: 1);
+    final previous = factory.created.single;
+
+    await session.selectLocked(1, preloadIndex: 2, previousIndex: 0);
+    await session.playbackSettled;
+
+    expect(session.active, isNull);
+    expect(previous.pauseCalls, greaterThan(0));
+    expect(previous.disposed, isTrue);
+    expect(factory.live, 0);
+    expect(factory.created, hasLength(1));
+
+    await session.select(2, previousIndex: 1);
+    expect(session.active, same(factory.created.last));
+    expect(factory.created.last.playCalls, greaterThan(0));
+    await session.close();
+  });
+
+  test('should_play_locked_episode_only_after_it_is_unlocked', () async {
+    final lockedEpisodes = [
+      Episode(
+        id: 'free',
+        number: 1,
+        title: 'Free',
+        isFree: true,
+        videoUrl: 'https://example.test/free.mp4',
+      ),
+      Episode(
+        id: 'locked',
+        number: 2,
+        title: 'Locked',
+        isFree: false,
+        locked: true,
+        videoUrl: 'https://example.test/locked.mp4',
+      ),
+    ];
+    final unlocked = <String>{};
+    final factory = FakeFactory();
+    final session = PlaybackSession(
+      episodes: lockedEpisodes,
+      factory: factory.create,
+      canPlay: (episode) => episode.isFree || unlocked.contains(episode.id),
+    );
+    await session.select(0);
+    await session.selectLocked(1);
+    expect(session.active, isNull);
+
+    unlocked.add('locked');
+    await session.select(1);
+
+    expect(session.active, same(factory.created.last));
+    expect(factory.created.last.playCalls, greaterThan(0));
+    await session.close();
+  });
+
   test('should_drop_stale_initialize_and_keep_one_player_when_selection_is_rapid', () async {
     final factory = FakeFactory(holdFirstInitialize: true);
     final session = PlaybackSession(