UDP: datagram struct + parsing

[benaadams]

Problem

Magma.Transport.Udp contains only a Header/ subdirectory and a .csproj file. No actual transport logic exists. UDP is the simpler of the two transport protocols and a hard dependency for DNS, QUIC, game networking, and most real-time use cases.

Start with the zero-copy datagram representation.

Sub-tasks

• UdpDatagram struct wrapping Span<byte> for zero-copy access to source/dest port, length, checksum, payload
• Construction methods: build a datagram into a pre-allocated buffer
• IPv4 pseudo-header checksum calculation (reuse Magma.Common)
• IPv6 pseudo-header checksum calculation

Dependencies

Depends on: #103 (establishes test patterns)
Unlocks: #107 (UDP transmitter/receiver), #106 (UDP tests)

Phase

Phase 1 (Core Protocol & Tooling)