Freeze audio to sample track

Nobody has claimed this yet.

## Enhancement Summary
This feature would add the ability to export audio from any audio track to a sample track in the project. It would likely be in the context menu for instrument tracks, the B+B editor, and other sample tracks. This feature would act like the already-existing feature that exports audio tracks, except it only exports one selected track, B+B pattern track, or sample track, and it would then load the saved file as a new sample block in a new sample track. An alternative method would be to simply export any group of soloed tracks and load the exported audio as a new sample. This would allow bouncing multiple tracks to one sample, but it would feel a bit less natural. If possible, the sample track should appear below the lowest selected track (or below the one track) that was frozen.

### Justification
This feature would be a workflow enhancement. It would be extremely helpful for reducing CPU usage for instruments or sounds that do use a lot of CPU. It could also be used to counteract plugin latency for plugins that add it. While this sort of thing is technically already possible, it is very inconvenient to do, and having a built-in tool would be fantastic for workflow.

No files, tests, or entry points are named. Start by locating the existing audio-track export workflow and the context menus for instrument, B+B, and sample tracks. Done means defining and implementing a consistent way to freeze the selected audio into a new sample track, including its placement and track-selection behavior.

Written by the indexing model from the issue text.