0004 — Batch limit pinned to 1

Context. Claiming several messages in one transaction ties their fates together: one poison message that fails or hangs poisons the whole batch, holding or rolling back messages that would have succeeded.

Decision. The claim batch limit is pinned to 1 — each transaction claims and processes a single message.

Consequences. Batch poisoning is impossible by construction. The cost is one claim round-trip per message, an accepted throughput ceiling at this scale.