Adds the systemd template for the credential-reuse correlator daemon
and wires it into decnet.target so `decnet init` installs it
automatically (the unit installer globs decnet-*.service.j2). Mirrors
the mutator template: bus-woken Type=simple service with the standard
hardening + on-failure restart.
Also registers `reuse-correlator` in the in-process worker registry
(so the dashboard panel surfaces its heartbeat instead of dropping it
as unknown) and slots it into the start-all preferred order between
mutator and webhook.