* fix(checkpoints): isolate cached history payloads from state readers
Copy nested snapshot seeds and pending-write payloads at both cache boundaries.
Keep user copy callbacks outside the registry lock so readers cannot poison
later checkpoint materialization or deadlock by re-entering the cache.
Cover sync/async state accessor reads, nested message payloads, disabled-cache
controls and callback lock ownership.
* docs(checkpoints): document synchronous history copy costs
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
