Delete audio checkpoints with no symlinks, and those over a certain age. This means they correspond to some older score state, so I can delete them and loading a new score will still likely hit the cache.

If I update mtimes on cache hits, the minimum age should keep alive the previous few generations, so undo continues to hit them.

A more sophisticated approach would be to keep an explicit generation count, so I could delete by generation, but this way is simpler and rerendering isn't a big deal.