Services diagnostics
Codes in the MGS5xxx range flag problems with long-running service
ops: services that will run as separate processes when they
look like they should be one shared instance, and (later) op invocations that
contradict their declared kind. Magus raises them at run time (as a typed
DiagnosticError) and, where applicable, as a magus doctor health check so the
gap is visible before the work runs.