The forget check and the store now happen in one Ref update, guarded by a
single forget counter, so a forget can't land between them and an older
answer can't come back. A failed request rechecks the last answer's age once
it settles, and entries too old to pace or stand in are pruned on each store.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>