fix: align HKDF info strings with featherChat identity derivation

Changed HKDF expand info strings to match featherChat's identity.rs:
- "warzone-ed25519-identity" → "warzone-ed25519"
- "warzone-x25519-identity" → "warzone-x25519"

Same BIP39 seed now produces identical Ed25519/X25519 keypairs in both
featherChat and WZP. This is the prerequisite for shared identity.

Also added FEATHERCHAT_INTEGRATION.md (1209 lines) from featherChat repo
documenting the full integration plan with confirmed code references.

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

crates/wzp-crypto/src/handshake.rs

@@ -33,13 +33,13 @@ impl KeyExchange for WarzoneKeyExchange {
         // Derive Ed25519 signing key via HKDF
         let hk = Hkdf::<Sha256>::new(None, seed);
         let mut ed25519_bytes = [0u8; 32];
-        hk.expand(b"warzone-ed25519-identity", &mut ed25519_bytes)
+        hk.expand(b"warzone-ed25519", &mut ed25519_bytes)
             .expect("HKDF expand for Ed25519 should not fail");
         let signing_key = SigningKey::from_bytes(&ed25519_bytes);
 
         // Derive X25519 static key via HKDF
         let mut x25519_bytes = [0u8; 32];
-        hk.expand(b"warzone-x25519-identity", &mut x25519_bytes)
+        hk.expand(b"warzone-x25519", &mut x25519_bytes)
             .expect("HKDF expand for X25519 should not fail");
         let x25519_static_secret = StaticSecret::from(x25519_bytes);
         let x25519_static_public = X25519PublicKey::from(&x25519_static_secret);