v0.0.38: FC-P4 complete — session versioning, wire envelope, auto-backup

FC-P4-T1: Session State Versioning
- RatchetState serialize/deserialize with [MAGIC:0xFC][VERSION:1][bincode]
- Legacy (raw bincode) still loads — backward compatible
- Client + WASM both use versioned format
- 2 new tests: roundtrip + legacy compat

FC-P4-T2: WireMessage Versioning Envelope
- Format: [WZ magic][version:u8][length:u32 BE][bincode payload]
- Server + client + WASM accept both envelope and legacy on receive
- Client still sends raw bincode (server handles both)
- Future version → "update required" error instead of crash
- 3 new tests: roundtrip, legacy compat, future version rejection

FC-P4-T3: Periodic Auto-Backup
- Every 5 minutes, encrypts sessions+contacts+sender_keys to ~/.warzone/backups/
- HKDF-derived key from seed, ChaCha20-Poly1305 AEAD
- Atomic writes (temp file + rename), rotates to keep last 3
- /backup command for manual trigger

127 tests passing (was 122)

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

warzone/crates/warzone-protocol/src/ratchet.rs

@@ -11,15 +11,20 @@ use crate::errors::ProtocolError;
 
 const MAX_SKIP: u32 = 1000;
 
+/// Current serialization version for [`RatchetState`].
+const RATCHET_VERSION: u8 = 1;
+/// Magic byte to distinguish versioned from unversioned (legacy) data.
+const RATCHET_MAGIC: u8 = 0xFC;
+
 /// A message produced by the ratchet.
-#[derive(Clone, Serialize, Deserialize)]
+#[derive(Clone, Debug, Serialize, Deserialize)]
 pub struct RatchetMessage {
     pub header: RatchetHeader,
     pub ciphertext: Vec<u8>,
 }
 
 /// Header included with each ratchet message.
-#[derive(Clone, Serialize, Deserialize)]
+#[derive(Clone, Debug, Serialize, Deserialize)]
 pub struct RatchetHeader {
     /// Current DH ratchet public key.
     pub dh_public: [u8; 32],
@@ -208,6 +213,37 @@ impl RatchetState {
         Ok(())
     }
 
+    /// Serialize with version prefix: `[MAGIC][VERSION][bincode data]`.
+    ///
+    /// Use [`deserialize_versioned`](Self::deserialize_versioned) to restore.
+    pub fn serialize_versioned(&self) -> Result<Vec<u8>, String> {
+        let data = bincode::serialize(self)
+            .map_err(|e| format!("serialize: {}", e))?;
+        let mut out = Vec::with_capacity(2 + data.len());
+        out.push(RATCHET_MAGIC);
+        out.push(RATCHET_VERSION);
+        out.extend_from_slice(&data);
+        Ok(out)
+    }
+
+    /// Deserialize with version awareness. Handles:
+    /// - Versioned format: `[0xFC][version][bincode]`
+    /// - Legacy format: raw bincode (no prefix)
+    pub fn deserialize_versioned(data: &[u8]) -> Result<Self, String> {
+        if data.len() >= 2 && data[0] == RATCHET_MAGIC {
+            let version = data[1];
+            match version {
+                1 => bincode::deserialize(&data[2..])
+                    .map_err(|e| format!("v1 deserialize: {}", e)),
+                _ => Err(format!("unknown ratchet version: {}", version)),
+            }
+        } else {
+            // Legacy: try raw bincode (pre-versioning data)
+            bincode::deserialize(data)
+                .map_err(|e| format!("legacy deserialize: {}", e))
+        }
+    }
+
     fn dh_ratchet_step(&mut self) -> Result<(), ProtocolError> {
         let their_pub = self
             .dh_remote
@@ -312,6 +348,35 @@ mod tests {
         assert_eq!(bob.decrypt(&m2).unwrap(), b"two");
     }
 
+    #[test]
+    fn versioned_serialize_roundtrip() {
+        let (mut alice, mut bob) = make_pair();
+        let msg = alice.encrypt(b"test versioning").unwrap();
+
+        // Save alice with versioned format
+        let serialized = alice.serialize_versioned().unwrap();
+        assert_eq!(serialized[0], 0xFC); // magic byte
+        assert_eq!(serialized[1], 1); // version 1
+
+        // Restore and use
+        let mut restored = RatchetState::deserialize_versioned(&serialized).unwrap();
+        let msg2 = restored.encrypt(b"after restore").unwrap();
+        let plain = bob.decrypt(&msg).unwrap();
+        assert_eq!(plain, b"test versioning");
+        let plain2 = bob.decrypt(&msg2).unwrap();
+        assert_eq!(plain2, b"after restore");
+    }
+
+    #[test]
+    fn legacy_deserialize_works() {
+        let (alice, _) = make_pair();
+        // Serialize with raw bincode (legacy format)
+        let legacy = bincode::serialize(&alice).unwrap();
+        // Should still deserialize with versioned reader
+        let restored = RatchetState::deserialize_versioned(&legacy).unwrap();
+        assert_eq!(bincode::serialize(&restored).unwrap(), legacy);
+    }
+
     #[test]
     fn many_messages() {
         let (mut alice, mut bob) = make_pair();