- regenerate docs/tools/hivectl-cli.md for the new `subvol snapshot send` verb
- close the TOCTOU on the no-overwrite guard: File::options().create_new(true)
(O_CREAT|O_EXCL) instead of exists()-then-create, so the guarantee is
atomic against a concurrent request racing the same dest filename
- warn (not silently swallow) if cleaning up a partial export after a
failed btrfs send itself fails, so a stuck garbage file masquerading
as a completed export is visible in the log
60a253a2f6