SessionEvents asked loginctl for -p ObjectPath, which is not a property — it
returned empty on every boot, so external lock signals were never monitored and
the shell had no unlock ingress. Resolve the graphical session the way
lockhint.rs does; not GetSession(auto) (this shell is a session-less user unit)
and not the /session/auto alias (PropertiesChanged only fires on the concrete
path).
SessiondBridge read root.registered to decide whether to re-register on
reconnect, but the disconnect branch had already cleared it, so the shell never
re-registered after a sessiond restart. Latch it.
4ed8f3dd74