UDP: delegate chain integration

[benaadams]

Problem

Wire UDP into the Magma.Network processing pipeline so it's usable end-to-end.

Sub-tasks

• Register UDP handler in the delegate chain (protocol number 17)
• Bidirectional: inbound demux and outbound frame construction
• Integration test: full stack from raw Ethernet frame through to UDP payload delivery

Dependencies

Depends on: #107, #104 (chain tests provide confidence)
Unlocks: #124 (UDP sample app), #131 (Kestrel integration), #137 (API docs)

Phase

Phase 1 (Core Protocol & Tooling)