Move design documents from working directory into source tree:
- fips-design.md - Full protocol design specification
- spanning-tree-dynamics.md - Spanning tree protocol dynamics study
Add README index files for docs/ and docs/design/.
Implements FIPS Identity System (Section 1 of design doc):
- NodeId: 32-byte SHA-256 hash of npub, with Ord for root election
- FipsAddress: 128-bit IPv6-compatible address (0xfd prefix)
- Identity: keypair holder with sign/verify methods
- AuthChallenge/AuthResponse: challenge-response authentication
All 11 tests passing.