On Linux, BSD-style `stat -f %m` prints a multi-line FILESYSTEM block to
stdout before exiting 1, so the BSD-first || chain captured that garbage
concatenated with the real `stat -c %Y` epoch. The non-numeric mtime made
`$(( now - mtime ))` a syntax error and set -e killed the binary with
exit 1 whenever a lock dir already existed — every contention path (stale
takeover, give-up, concurrent writers) broke on CI while staying green on
macOS, where BSD stat -f succeeds cleanly.
GNU `stat -c %Y` now goes first (BSD stat rejects -c with no stdout, so
macOS falls through cleanly), and a numeric guard blanks any residual
garbage so a future platform quirk degrades to the normal give-up path
instead of an arithmetic abort. Same defect class as gstack-repo-mode's
GNU-first ordering (#2195). Verified in an oven/bun Linux container:
the four CI-failing lock tests now pass (62/62 across both files).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
