docs: Architecture guide

[benaadams]

Problem

No document explains how the components compose. Contributors need this to understand where new code fits.

Sub-tasks

• Create docs/ARCHITECTURE.md
• Diagram: OSI layer mapping to Magma projects (Mermaid or SVG)
• Explain the delegate-based packet processing chain in Magma.Network
• Document the IPacketTransmitter / IPacketReceiver contract
• Describe how platform backends (NetMap, AF_XDP, WinTun) plug into the abstraction

Dependencies

Depends on: nothing
Unlocks: #114 (getting started tutorial), helps all implementation work

Phase

Phase 0 (Foundation) - can start immediately