feat: Phase 3 Week 1 — X3DH key agreement + HKDF foundation
- Add hkdf = "0.12" dependency for Double Ratchet key derivation
- Extend mesh/crypto.rs with hkdf_sha256, hkdf_sha256_32, hkdf_sha256_64,
and generate_x25519_ephemeral() for DH ratchet steps
- Create mesh/x3dh.rs: full X3DH key agreement protocol
- PrekeyBundle generation with Ed25519-signed prekeys
- 3-way (or 4-way) ECDH → HKDF-SHA256 → root key
- Initiator and responder sides derive identical root key
- CBOR encoding for mesh transmission
- Bundle signature verification
- 5 unit tests: generate+verify, both-sides-same-key,
without-one-time-prekey, cbor-roundtrip, tamper-detection
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
37a591618d