feat(deploy): systemd unit for decnet-reuse-correlator

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.

deploy/decnet.target

@@ -13,6 +13,7 @@ Wants=decnet-bus.service \
       decnet-sniffer.service \
       decnet-prober.service \
       decnet-mutator.service \
+      decnet-reuse-correlator.service \
       decnet-webhook.service
 After=decnet-bus.service