_user_systemd_socket_ready() accepts systemd/private alone, which is enough for
systemctl --user but not for the systemd-run --user that restart-safe workers
need; systemd_user_bus_env() requires the bus socket. Replace the uid threading
through five helpers with one _wait_for_target_user_bus(uid) that polls
/run/user/<uid>/bus, and move the post-enable wait + restart hint out of
_ensure_linger_enabled into _ensure_system_service_linger so the activity probe
runs only when linger was actually just enabled. Kanban applies the bus env
unconditionally like the cron sibling. Refs #104893.
6f11a3296e