Async retain already keeps the memory WRITE off the reply path (writes drain
on the single writer thread while the user gets their response immediately).
This closes the remaining retain/prefetch race: the next turn's warm prefetch
runs on its own thread and could recall BEFORE the just-enqueued retain write
lands, silently dropping the latest turn from recall.
- The background prefetch now waits (bounded) for pending retains to drain
before recalling, so warmed context includes the just-completed turn.
- The wait runs only on the background prefetch thread, never the reply path,
so it adds zero latency to the user's response and loses no writes.
- Bounded by prefetch_retain_drain_timeout (default 10s) and polls
unfinished_tasks so a wedged write can't hang the prefetch.
- New config keys: prefetch_waits_for_retain (default true),
prefetch_retain_drain_timeout (default 10.0).
And
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