The grants part composes the bus's user list with the composer the controller uses (ADR 0547 §5), copied rather than
moved: the controller is not changed and keeps its own until it loses the lease. Left out: the bus client, the broker's
address from the environment and the client's two methods; the lease's two subjects are copied in.
One table test holds the copy to four meshes' lists and the server golden, written by the controller's own
internal/broker from the same fixtures (testdata/controller-user-lists.json).
