refactor(intra): migrate UDP path from PacketProxy to packetrelay

network.PacketProxy and NewPacketProxyFromPacketListener are deprecated in
favor of the flow-based network/packetrelay API. This migrates Intra's UDP
handling, mirroring OutlineFoundation/outline-apps#2777.

- intraPacketProxy becomes intraPacketRelay, a packetrelay.PacketRelay whose
  associations share a dohAssociation between their sender and receiver halves.
- lwip2transport.ConfigureDevice -> ConfigureDeviceWithRelay.
- packetrelay.NewPacketRelayFromPacketListener replaces
  network.NewPacketProxyFromPacketListener, keeping the same 5 minute
  write-idle timeout (now a constructor argument).
- Add unit tests for DNS interception, packet relaying, byte accounting and
  sender close semantics.

Behavior is unchanged, except that SendPacket on a closed association now
returns ErrClosed instead of issuing a DoH query whose response can no longer
be delivered.

Author: fortuna