Issue 173's rule needs it: a mail provider serves its domain and an identity provider its issuer,
and neither is the definition's to state. Declared as ${setting:<key>}, filled from the layers after
the overrides; a key so asked for is not stray.
Three of novox/hq's group-4 leftovers, one branch.
Issue 173: a module's settings reached every route it contributed, every database it asked for and
every served fact its consumers read — a mail server's site name arrived at the proxy as a route
fact. A setting now overrides a key a contribution or served fact declares and adds none; a file
still merges any key, and a key nothing takes is named as stray instead of dropped silently.
Issue 174: the mesh's own files for a module — its bus credential, its merged config, its bindings —
were placed by the definition under /var/lib/mesh/<module>, 232 host paths in 50 definitions. A
directory may now say `place: "mesh"` and resolves to <root>/mesh/<module>; a directory beneath a
placed one may state its path as `${dir:<id>}/<rest>` and moves with it. The proof test resolves
both catalogues and compares: 48 definitions, no path moved. The controller's own manifest is
converted here; the catalogue in mesh-catalog.
ADR 0155: the installation check moves to registration. `module add` and a build's result both
refuse a definition that names an installation, in the check's words, with the way out; the build
stays recorded.
jschoubben