scan takes an exclusive flock(2) on a lock file beside the database
(its path with .lock appended) before it walks anything or opens the
database, and holds it until it returns. A second scan against the
same database fails at once with a one-line error naming the lock
file and exits 1. report and trees never take the lock. The lock ends
with the process, so a fatal error or an interrupt releases it; the
file is never deleted. golang.org/x/sys becomes a direct dependency.
The README smoke test now keeps the database outside the scanned
tree, where its empty lock file would have joined the empty-file
group.
Model: opus-5-5
sneak