Concept-Check: Persistence

This is where SQL stopped being a filing cabinet and started being the thing that keeps the coordinator honest under concurrency. A run now has a durable home; the atomic claim guarantees two schedulers never grab the same run; the transaction makes "job done" and "run counter moved" one indivisible fact; and the idempotency guard makes recording the same outcome twice count once — the store-side half of the at-least-once safety the cluster arc pays off. If these are solid, the service and scheduler arcs have a persistence layer they can trust when machines fail.