feat: featherChat-compatible identity — seed, mnemonic, fingerprint

New identity module (wzp-crypto/src/identity.rs) mirrors featherChat's
warzone-protocol identity.rs exactly:
- Seed: 32 bytes, from hex or BIP39 mnemonic (24 words)
- HKDF derivation: same salt (None), same info strings
- Fingerprint: SHA-256(Ed25519 pub)[:16], same xxxx:xxxx format
- Cross-verified: test proves identity module matches KeyExchange trait

CLI flags:
- --seed <64 hex chars>: use a specific identity
- --mnemonic <24 words>: use BIP39 mnemonic from featherChat
- Without either: generates ephemeral identity

Also adds featherChat as git submodule at deps/featherchat for reference.

32 crypto tests passing (27 original + 5 identity tests).

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

crates/wzp-crypto/src/lib.rs

@@ -9,12 +9,14 @@
 
 pub mod anti_replay;
 pub mod handshake;
+pub mod identity;
 pub mod nonce;
 pub mod rekey;
 pub mod session;
 
 pub use anti_replay::AntiReplayWindow;
 pub use handshake::WarzoneKeyExchange;
+pub use identity::{Fingerprint, IdentityKeyPair, PublicIdentity, Seed};
 pub use nonce::{build_nonce, Direction};
 pub use rekey::RekeyManager;
 pub use session::ChaChaSession;