One sample whose stream never got a first byte (prefill queue at high
gate levels) exhausted 6 adapter retries and killed the WHOLE gather --
500 samples died with it. Terminal failures are now contained: empty
prediction (scores wrong, es-parity for timeouts), NOT checkpointed so
a rerun retries them, prominently counted; only a 100% wipeout fails
the bench. Stream-aggregate read timeout 60s -> 240s: a 128k prompt
queued behind other prefills legitimately takes minutes to start
answering; real hangs are now the gate's job (x0.7) and contained
failures rather than bench death.
Verified: 1-in-3 terminal failures -> bench completes 6 ok / 3 empty,
checkpoint holds only the 6.
Co-Authored-By: Claude <noreply@anthropic.com>