Function is_available 

pub fn is_available() -> bool

Whether a systemd user manager is running and can host the daemon.

A fast, subprocess-free probe: the system must be booted with systemd (/run/systemd/system exists) and the current user’s manager control socket ($XDG_RUNTIME_DIR/systemd/private, defaulting to /run/user/<uid>/systemd/private) must be present. Returns false under the OMNI_DEV_DAEMON_DISABLE_SYSTEMD escape hatch. When this is false the caller falls back to a detached daemon run spawn.