Function bind_first_unix_socket 

pub fn bind_first_unix_socket(socket_path: &str) -> Result<UnixListener>

Unix counterpart of bind_first_pipe_instance: bind a Unix domain socket. The bind is the single-instance signal — a second launcher pointing at the same socket path gets EADDRINUSE. Caller is responsible for unlinking a stale socket file first (see the connect → ECONNREFUSED → unlink → bind pattern in main.rs::run_unix).

A1.1 of SPEC_LAUNCHER_LINUX_PACKAGED_AND_SPLASH_2026_06_05.