Preparation for running the ML-KEM exchange on the js/wasm client. There, and
in other netstack-mode clients, the overlay IP exists only inside the
userspace stack, so the KEM socket has to be opened on the netstack. The
netstack returns a *gonet.UDPConn rather than a *net.UDPConn, so the
transport can no longer hold the host socket's concrete type.
Hold the socket as a net.PacketConn, which both implement, and convert
endpoints at the edges. Sources are unmapped explicitly, since a udp4
socket's net.UDPAddr may carry the 16-byte form. No behaviour change: the
transport still binds on the host, and the next commit opens it on the
netstack. Adds a host round-trip test that also covers the fallback to an
ephemeral port.
riccardom
1152b25149