Two overlapping `omp update` runs now share the target only for the
swap + stale-artifact sweep, guarded by withFileLock. This closes the
rollback race the unique-temp fix left open: a concurrent run's sweep
could delete a live run's .bak before its failed verification rolled it
back. The download stays outside the lock (unique temp path, safe to
overlap). Adds a regression covering a failed verification overlapping a
successful update.
can1357
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