[WIP] Add test coverage for UDP transport

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>test: UDP transport</issue_title>
<issue_description>## 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)</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes test: UDP transport #106

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