播放缓存视频，只有音轨，视频白屏

猜测可能是缓存的alv文件异常，没有缓存完成的时候，会出现这种情况。

Start by reproducing playback with an incomplete cached ALV file and compare it with a fully cached file. Trace the cache and playback paths to determine why audio continues while the video is blank. Done means the incomplete-cache case is handled without producing audio-only playback, with the behavior verified on a reproducible test case.

Written by the indexing model from the issue text.