Write falls through to a real write when the liveness touch fails, and sweep retention now matches the recorded mime's extension so obsolete variants are reclaimed.
Duplicate ImageStore writes refresh the file mtime and Remove skips files newer than the cutoff, so overlapping acquisitions cannot lose their store files to a concurrent prune.
Sweep now honors an mtime grace window (in-flight acquisitions and temp files), reacquired orphans reset the prune grace window, and the queue mock implements real stale-absent semantics.
