guard-cargo#m5 pv-pin: nonzero 'pv --version' swallowed to empty and reported as STALE (MG 39236323675) #5755

Owner: aprender-flow. Gate: guard-cargo#m5 (x86-main) on the merge-queue path.

MG 39236323675 (head #5734, runner intel-clean-room-4, 11:15:03Z): pv-pin pin failed from a subdirectory (rc=1); resolved snapshot pv reports : EMPTY, declares 0.71.3. The same snapshot path resolved fine in the two cases just before it.

Root cause (code, measured): scripts/pv_bin.sh:684 pv_bin_vers=$("$pv_bin_b" --version 2>&1) || pv_bin_vers="" turns any nonzero exit (kill/OOM/ETXTBSY/timeout) into an empty string, which is then reported as "STALE pv BINARY". Same swallow at :369, :458, :501.
NOT MEASURED: snapshot race vs pv killed under intel load 185/32 (no rc kept).

Fix (two-way door): capture rc+stderr; rc!=0 -> "pv --version did not answer (rc=N)" classed RUNNER, retry once; STALE only when rc==0 and version differs. Case table + mutation. Write-up: cop-inbox handoff/pvpin-m5-rootcause-1115Z.md.