Same contract as the 27 repaired byte/artifact survivors: checked
acquisition, one all-exit cleanup funnel per carrier, ENOENT-tolerant
checked unlinks, exact-path deletion, and cleanup failure fails a
passing carrier without overwriting its diagnostic. Also closes the
vacuous-green channels: empty-artifact pairs no longer byte-compare
equal, capture and staging failures fail their row loudly, 950 requires
a clean checker exit on no-error rows and hard-fails on a missing
wwdump_ww, 992 keys its scratch per invocation so a failed cleanup
cannot leak stale objects into the next tool's compare, and 995 no
longer pre-sweeps a workdir it never created. No assertion is weakened;
990_selfhost is retired in the next commit rather than repaired.
Hojun-Cho
cdc8bda721