perf(player): 续看点加载与进度写盘改为并行且复用同一请求

进播放页与首次准备播放会各要一次续看点，同一次请求跑两遍还串在首帧前；
现在并发请求复用同一 Future，并与上一条进度写盘并行等待，少一个网络往返的起播延迟。
剧场网格海报按布局宽度限制解码尺寸，半屏卡片不再按原图解码。

test/playback/progress_throttle_test.dart

@@ -158,6 +158,18 @@ void main() {
       logic.dispose();
     });
 
+    test('should_share_one_request_when_resume_point_loads_concurrently', () async {
+      final history = FakeWatchHistory()..saved = const Duration(seconds: 20);
+      final logic = _logic(history, drama: _drama('signal'));
+
+      // 进页与首次准备播放会同时要续看点，只应发一次请求。
+      await Future.wait([logic.loadResumePoint(), logic.loadResumePoint()]);
+
+      expect(logic.resumeLoaded, isTrue);
+      expect(history.reads, ['signal']);
+      logic.dispose();
+    });
+
     test('should_not_seek_when_position_is_zero', () async {
       final history = FakeWatchHistory()
         ..records['signal'] = const WatchProgress(