UDP: transmitter + receiver

[benaadams]

Problem

The actual UDP transport plumbing - implementing the IPacketTransmitter and IPacketReceiver interfaces for UDP.

Sub-tasks

• UdpTransportReceiver implementing IPacketReceiver - accept raw IP packets, extract UDP datagrams, demux by port
• UdpTransportTransmitter implementing IPacketTransmitter - accept datagrams, construct IP+UDP frame, hand to network layer
• Port binding / socket table for demuxing incoming datagrams

Dependencies

Depends on: #105, #106
Unlocks: #108 (UDP delegate chain integration)

Phase

Phase 1 (Core Protocol & Tooling)