test: 15 cross-project integration tests — WZP ↔ featherChat verified

Identity (6 tests):
- Same seed → same Ed25519/X25519 keys, same fingerprint, same display
- Random seed, raw HKDF output verified

BIP39 Mnemonic (3 tests):
- Roundtrip both directions, identical strings

CallSignal Interop (4 tests):
- Offer/Answer/Hangup roundtrip through FC bincode serialization
- Signal type mapping verified

Auth Contract (2 tests):
- Request/response shapes match between WZP and FC

Uses warzone-protocol v0.0.21 as real dependency.

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

crates/wzp-crypto/Cargo.toml

@@ -18,4 +18,15 @@ tracing = { workspace = true }
 bip39 = "2"
 hex = "0.4"
 
+# featherChat identity — the source of truth for Seed, IdentityKeyPair, Fingerprint
+warzone-protocol = { path = "../../deps/featherchat/warzone/crates/warzone-protocol" }
+
 [dev-dependencies]
+ed25519-dalek = { workspace = true }
+warzone-protocol = { path = "../../deps/featherchat/warzone/crates/warzone-protocol" }
+wzp-proto = { workspace = true }
+wzp-client = { path = "../wzp-client" }
+wzp-relay = { path = "../wzp-relay" }
+serde_json = "1"
+serde = { workspace = true }
+bincode = "1"