feat: add crypto module (Ed25519 key generation, sign, verify)

Moves verifySignature, signMessage, generateKeyPair and utility functions
from Yonexus.Client into Protocol so Server no longer depends on Client
at build time or runtime.

src/index.ts

@@ -2,3 +2,4 @@ export * from "./types.js";
 export * from "./codec.js";
 export * from "./errors.js";
 export * from "./auth.js";
+export * from "./crypto.js";