docs: comprehensive project documentation
- ARCHITECTURE.md: protocol design, wire format, FEC, crypto, relay modes
- USAGE.md: build instructions, all CLI flags, deployment examples
- DESIGN.md: rationale for codec/FEC/transport/crypto choices
- EXTENSIBILITY.md: trait extension points, Warzone integration, future features
- PROGRESS.md: phase 1-4 timeline, test coverage, known issues
- API.md: complete crate API reference for all 8 crates
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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