buffer_copy_mono reverse behavior with overlapping regions

[catfact]

this might be known behavior that just needs clarification, idk.

reported on lines:
https://llllllll.co/t/norns-2-0-softcut/20550/281

i'm not exactly clear on what is going on but it seems like maybe unintended results are happening when destination overlaps source (such as attempting to reverse in place.) or maybe it is known that the operation will be truncated as needed in order not to allocate working memory.

that logic is here:
https://github.com/monome/norns/blob/main/crone/src/BufDiskWorker.cpp#L253-L279