The foundation's revUpdateJob/revUpdateLease did a read -> rev-check -> write with
await points between them, so two CONCURRENT claims could both read the same rev,
both pass the check, and both write — a double-assignment the old (sequential) race
test could not catch.
Rewire revUpdateJob/revUpdateLease to delegate to the datastore's updateIfMatch,
which performs the compare and the write as one indivisible operation (Cosmos
If-Match; synchronous compare-set on memory). The coordinator's tryClaimJob keeps
identical external behavior (ok/conflict) but is now genuinely single-winner.
Upgrades the coordinator tests to prove atomicity under TRUE concurrency:
- two contenders via Promise.all -> exactly one ok, one conflict; assigned once;
one run; one lease; leaseEpoch 1.
- N-claimer (15) stress via Promise.all -> one ok, N-1 conflicts, no double-assignment.
- N concurrent claimNextJob for one job -> exactly one non-null claim.
- N concurrent lease renewals -> exactly one wins.
Verified these concurrent tests FAIL against the old read-check-write (double-assign)
and pass after the fix.
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