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>

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "deps/featherchat"]
+	path = deps/featherchat
+	url = ssh://git@git.manko.yoga:222/manawenuz/featherChat.git