Until now a membership — bus address, fingerprint, password — was written once,
at enrolment, and nothing ever rewrote it, so a machine already enrolled could not
be moved to another bus at all (novox/hq design 28, task 5.2). The mesh now
delivers a membership for the new bus as a sealed file in the declaration, like
any secret; the host reads it after the declaration has applied, saves it as its
identity, and exits cleanly so the service manager restarts it dialling the bus it
names. The same path a machine takes after a reboot — so nothing new has to be
right for it to work. A membership carries its transport; empty means the bus the
mesh ran on before, so nothing written earlier reads as unset.