[Queue] Implement transactional dispatch, leases, deduplication, and fencing #32

Outcome

Deliver accepted work durably while allowing safe worker replacement and duplicate messages.

Scope

• Implement queue abstraction, outbox dispatch, acknowledgement/visibility semantics, lease renewal/expiry, per-resource serialization, fencing, and bounded concurrency.

Acceptance criteria

• Lost acknowledgements and repeated dispatch do not lose work.
• Crashed-worker leases recover and stale workers cannot commit.
• Poisoned work reaches a bounded, inspectable terminal path.

Test and evidence

• Duplicate, lease-race, dispatcher-crash, queue-outage, and poison-message tests.

Dependencies

• Blocked by #31.
• Blocked by #12.

References

• Architecture overview