split file duration is wrong

I've tested the script, it works fine, with one issue. Every single file has a total runtime of 2 hours and 24 minutes for me. The source YouTube video is indeed 2 hours and 24 minutes long.

Perhaps a metadata issue, or a config issue on my end maybe? I'll include my youtube-dl config to be sure:

```
-x
--audio-format=flac
--audio-quality=0
--restrict-filenames
-o ~/Downloads/%(title)s.%(ext)s
```

Start by reproducing the split using the reported youtube-dl configuration and inspect how the script reads video and chapter metadata. Trace why each output file receives the full source duration; the issue is done when each split file reports its own chapter duration.

Written by the indexing model from the issue text.