Function register_nav 

pub fn register_nav(ctrl: NavController) -> NavToken

Install a controller (innermost = last). Returns its token. The root nav() registers once and never unregisters; nested hosts (tabs() in a route) unregister when their scope disposes.

If an absolute navigation left unconsumed segments, the new surface consumes as many leading ones as it accepts — this is how navigate("mail/inbox/msg-42") reaches a stack that only mounts once the “mail” switch has taken effect.