v0.0.13: Sender Keys for efficient group encryption

Protocol (sender_keys.rs):
- SenderKey: symmetric key with chain ratchet (forward secrecy per chain)
- generate(), rotate(), encrypt(), decrypt()
- SenderKeyDistribution: share key via 1:1 encrypted channel
- SenderKeyMessage: encrypted group message (O(1) instead of O(N))
- Chain key ratchets forward on each message (HKDF)
- Generation counter for key rotation tracking
- 4 tests: basic, multi-message, rotation, old-key rejection

WireMessage:
- GroupSenderKey variant: encrypted group message
- SenderKeyDistribution variant: key sharing

Server: dedup handles new variants.
CLI TUI + recv: stub handlers for new message types.
23/23 protocol tests pass.

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

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

@@ -84,4 +84,21 @@ pub enum WireMessage {
         total_chunks: u32,
         data: Vec<u8>,
     },
+    /// Group message encrypted with sender key (O(1) instead of O(N)).
+    GroupSenderKey {
+        id: String,
+        sender_fingerprint: String,
+        group_name: String,
+        generation: u32,
+        counter: u32,
+        ciphertext: Vec<u8>,
+    },
+    /// Sender key distribution: share your sender key with a group member.
+    /// This is sent via 1:1 encrypted channel (wrapped in KeyExchange/Message).
+    SenderKeyDistribution {
+        sender_fingerprint: String,
+        group_name: String,
+        chain_key: [u8; 32],
+        generation: u32,
+    },
 }