Offloading the fsync to a worker thread (previous commit) lets two inbound
messages have two persists in flight. Without ordering, a slow first flush
lands after the second and the disk file loses the newer token/id/seen-id
(in-memory state stayed correct, so the loss only surfaced after restart).
The loop-side dict could also be mutated while the worker iterated it.
- weixin ContextTokenStore / discord non-conversational tracker: snapshot
the payload on the loop, hand it to the worker, serialize with a per-store
asyncio.Lock.
- feishu dedup: same asyncio.Lock around the offloaded persist (its snapshot
was already taken under _dedup_lock; the write was not).
- Regression tests: two concurrent persists with a slow first write must
leave the union on disk. All three fail on the previous commit.
1730a2c32a