Function bind_socket_registered 

pub fn bind_socket_registered(
    root: &Path,
    run_dir: &Path,
    reg: &RegistrationFile,
) -> Result<(LocalListener, SocketEndpoint)>

Bind the owner tree’s socket and publish reg for it in one call, for a daemon that knows its surface at start.

A refused registration fails the call: a socket no reader can find is not the outcome the serving side asked for.