test: AF_XDP integration

[benaadams]

Problem

Magma.AF_XDP has no tests. It's the recommended Linux backend and needs validation.

Sub-tasks

• Create test/Magma.AF_XDP.Facts project (Linux-only, skip on Windows)
• Mock-based unit tests for XDP socket setup, UMEM configuration
• Integration test strategy: requires XDP-capable NIC or veth pair, gate behind test category
• Docker setup for CI with XDP support (may need privileged container)

Dependencies

Depends on: #99 (Linux CI)
Unlocks: #120 (AF_XDP TCP echo server sample)

Phase

Phase 2 (Platform Backends & Samples)