A cached rewrite made with cooldown on stays right until a version it held
back leaves cooldown, so it now expires then instead of after a fixed
time. Handlers ask cooldown through Proxy.cooldown, which reports each
withheld version to the rewrites running at that moment. A rewrite that
held nothing back is kept for the shortest cooldown, at most a day.
A response arriving during a hold was sent before it began, so a late
success no longer releases the host; only one after the hold has ended
does.
Artifact fetches check RateLimitGate.Held before calling the fetcher, so
held cache misses fail with the HeldError without counting as failures
in the host's circuit breaker.
Every handler that answers an upstream fetch error with 502 now answers
a held request with 503 and Retry-After instead.
OnHold and RecordUpstreamRateLimitHold drop the unused duration.
