Review follow-up on the #62871 salvage (simplify pass, HIGH):
1. Ops unresolved at the wait deadline were RETAINED in the pending set.
A permanently failing status endpoint (auth error, endless 500s, or a
server that loses ops without 404) would grow the set forever and make
EVERY later prefetch burn the full 10s budget re-polling it — and
prefetch()'s bounded 3s join sits on the reply path, so that money-quote
'adds no response latency' claim breaks. Timed-out ops are now dropped
(identical degradation to prefetch_waits_for_retain=False: possibly
stale recall) with a WARNING so persistent server trouble is visible.
Guard test mutation-checked (fails with eviction disabled).
2. Status polls now spaced 0.5s (was 0.05s shared with the local drain
poll): a wedged op cost up to ~200 get_operation_status round trips
per prefetch; now ~20 max over the default 10s budget.
9bbd956b73