feat: add per-peer ratchet session manager with disk persistence

- 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>

core/archipelago/src/mesh/mod.rs

@@ -17,6 +17,8 @@ pub mod types;
 #[allow(dead_code)]
 pub mod ratchet;
 #[allow(dead_code)]
+pub mod session;
+#[allow(dead_code)]
 pub mod x3dh;
 
 pub use types::*;