feat: Phase 3 Week 2 — Double Ratchet protocol for forward-secret mesh messaging

- Create mesh/ratchet.rs: full Signal-style Double Ratchet implementation
  - DH ratchet with X25519 ephemeral keypairs per step
  - Symmetric-key ratchet via HKDF-SHA256 chain derivation
  - Per-message ChaCha20-Poly1305 encryption with derived message keys
  - Out-of-order delivery via skipped message key cache (max 100)
  - Forward secrecy: old keys zeroized on ratchet step
  - Wire format: 40B header + nonce + ciphertext + tag
- Tests: full conversation, out-of-order, forward secrecy, wire format,
  long conversation (50 messages alternating), message roundtrip

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/mesh/mod.rs

@@ -15,6 +15,8 @@ pub mod serial;
 #[allow(dead_code)]
 pub mod types;
 #[allow(dead_code)]
+pub mod ratchet;
+#[allow(dead_code)]
 pub mod x3dh;
 
 pub use types::*;