0422 — Exclusivity is claim-per-instance, not claim-per-tick

Context. The old duty table enforced exclusivity claim-per-tick: a conditional UPDATE raced on every interval, so the pacing of the loop and the ownership of the loop were the same mechanism.

Decision. A worker claims its worker_instance slot once at Register; the heartbeat-renewed expires_at holds it from then on. The heartbeat IS the exclusivity. Tick pacing becomes the worker’s own internal business.

Consequences. Ownership arbitration happens once per instance lifetime instead of once per tick. The cost is lease-shaped failover: a dead instance’s slot stays occupied until its lease expires, so takeover waits on the TTL rather than the next tick. Rejected: claim-per-tick — it couples exclusivity to pacing and re-runs arbitration on every interval.