nx_socket_pingpong_test.nx

nx_socket_pingpong_test.nx -- prove TCP listen/accept/connect loopback works via real Linux syscalls. Foundation for L9 distributed MIMD (nx_net_chan, nx_remote_spawn, nx_rpc). Design: parent thread is the SERVER (listens + accepts + echoes "ping" -> "pong" once + closes); a spawned child thread is the CLIENT (sleeps briefly to let parent bind, then connects + sends "ping" + receives "pong" + closes + sets done flag). Main spin- waits on the done flag. Uses port NX_PORT (chosen high to avoid privileged + commonly-used); if port is already in use the bind() returns -EADDRINUSE which we surface as a clear failure code rather than hanging.