- https://github.com/Vanilagy/mp4-muxer/issues/83 tell him it's possible now
- textsubtitlesource, chunked piping
- More efficient MP3 loading when reading sequentially
- Redo readers: I think better to have a single reader with ephemeral/non-ephemeral loading methods than to have multiple readers. This way, they can still share data.
- Readers: Does it make more sense to have `pos` handled outside? Otherwise it's ripe for async race conditions.

For release:
- Write README
- Migration guides on old repos & deprecation
- llms.txt???