feat(messagequeue)!: drop the requeue delay parameter from Nack

[behinddwalls]

Summary

Why?

Nack's delay parameter was vestigial. The consumer framework — the only production caller — always passed 0 and let the visibility timeout space retries, and the delay's other conceivable use ("check back later" pacing) is exactly what Postpone now expresses with correct retry accounting. Keeping the parameter left two overlapping delay knobs on the delivery contract and invited nack-as-backoff, which burns the DLQ budget.

What?

Delivery.Nack becomes Nack(ctx): the message is immediately eligible for redelivery, retries are spaced by the visibility timeout, and the redelivery counts toward the failure budget as before. MarkNacked drops its delay accordingly (invisible_until = now). Integration tests: TestNackWithDelay is deleted (the delay was the feature; redelivery-after-lapse is already covered by TestVisibilityTimeoutAndRetry); TestNackDoesNotBlockOtherMessages becomes TestInFlightMessageDoesNotBlockOtherMessages, proving the skip property with an un-finalized in-flight head instead of a 30s nack (arrange-first, mirroring the postpone barrier test it contrasts with); other call sites are mechanical. Docs updated (READMEs, sql-queue RFC nack semantics).

Test Plan

✅ make test (83 targets). ✅ bazel test //test/integration/extension/messagequeue/... — DLQ at MaxAttempts via immediate nacks, non-blocking in-flight head, multi-consumer-group independence. ✅ make fmt, make mocks.

Stack

1. docs(rfc): consumer hold — postpone a delivery as a partition barrier #486
2. feat(platform): hold/postpone — a fourth delivery outcome for backing off #487
3. feat(stovepipe): migrate process and buildsignal waits to the hold primitive #488
4. feat(orchestrator): migrate the buildsignal poll loop to the hold primitive #489
5. feat(messagequeue)!: remove PublishAfter and the visible_after column #490
6. @ feat(messagequeue)!: drop the requeue delay parameter from Nack #491
7. feat(consumergate)!: rebuild the gate on the postpone primitive #492