mesh/merge-gate pass: builds build-agent, mesh-controller, route-proxy → ace, g14, novox, shanks; no bus step; every machine composes with the change as it…
The keeper keeps each transition from its own goroutine, which reads the memory
store's Fail field under the store's lock; tests assigned the exported field
bare, so TestAnUnreadableStoreClearsNothing failed under -race whenever the
goroutine appended in that window. The field is now set only through SetFail
(and Told's likewise), and a test makes the race certain rather than rare.
Test-only: the controller runs the bus store, never InMemory.
