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>
