Adds NetSocket (non-blocking UdpSocket wrapper with local_addr), NetServer (connection tracking via HashMap, poll/broadcast/send_to_client), and NetClient (connect/poll/send/disconnect lifecycle). Includes an integration test on 127.0.0.1:0 that validates ClientConnected, Connected, and UserData receipt end-to-end with 50ms sleeps to ensure UDP packet delivery. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>