idempotency.sx: persist/append-once appends at most once per (stream,
idempotency key), returning the same event on a repeat. The marker lives in the
kv facet, so idempotency holds across a restart (verified on durable).
persist/seen? check. 180/180.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
