Default transport: one BEAM-created Unix-domain socket per worker.
The BEAM listens before the worker is launched (FR-1); the worker
connects once, using the socket path passed as a plain command-line
argument (never shell-interpolated). Delivers data as standard
:gen_tcp active-mode messages ({:tcp, socket, data},
{:tcp_closed, socket}, {:tcp_error, socket, reason}) to whichever
process calls accept/2, so a GenServer can handle_info/2 them
directly.