- proxy/engine: NetworkEngine that owns the smoltcp VirtualNetwork and
bridges async TCP streams to virtual sockets via a 5ms poll loop.
Each ProxyConnection holds the local TcpStream + smoltcp socket
handle and shuttles data between them with try_read/try_write so the
engine never blocks.
- proxy/tcp: skeleton TcpProxy listener (currently unused; the daemon
inlines its own listener that hands off to the engine via mpsc)