Length-framed stdin/stdout fallback transport, used automatically only
on platforms where Unix-domain sockets are unsupported. Experimental:
Linux (the only officially supported 1.0 platform) always uses
Portals.Transport.Unix.
Unlike the Unix transport, there is no separate listen/accept step: the
worker's stdin/stdout is the connection, opened by
Portals.WorkerLifecycle.Port with the {:packet, 4} port option, which
gives the same length-prefixed framing as the Unix transport's packet: 4 socket option. listen/1 and accept/2 are no-ops that simply
adopt the already-open port.