test: UDP transport

[benaadams]

Problem

UDP transport needs test coverage as it's being built.

Sub-tasks

• Create test/Magma.Transport.Udp.Facts project, add to solution
• Datagram construction round-trip: build, serialise, parse, verify fields
• Checksum computation: validate against known UDP packets
• IPv4 vs IPv6 pseudo-header correctness
• Edge case: zero checksum (valid for UDP over IPv4), minimum-length datagram

Dependencies

Depends on: #105
Unlocks: #107 - confidence to build transport logic

Phase

Phase 1 (Core Protocol & Tooling)