ADR 0121. The builder declared `built` as its own event, so every consumer depended
on which module happens to be the build machine today. It is the build-machine
role's event now: the builder declares none of its own, and the catalogue listens
for `mesh-build-machine.built` rather than `builder.built`.
Nothing changes about what reaches the catalogue. What changes is that it survives
the build machine being a different module, which is the whole reason the mesh has a
word for a role.