- Create mesh/session.rs: SessionManager for Double Ratchet state lifecycle
- Lazy-loads sessions from disk on first message
- Saves after every encrypt/decrypt (chain key advancement)
- Per-DID storage at {data_dir}/ratchet/{sha256(did)}.json
- Session info API for RPC status reporting
- Zeroize on drop for all key material
- Tests: store+load roundtrip, encrypt/decrypt through manager, session removal
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>