BS04 gate (2c, Low): the post-admission stale-snapshot abort released the
lease and returned without emitting the aborted-attempt telemetry, so a
stale manual /compress never reached the compression_attempt log line or
the success/failed counter (the thread's pending attempt was left open).
Emit `snapshot_stale` like the other abort paths, and assert the log line
in the stale cases of the existing fence test.
c9d0d5fef8