fix: initialize SHA-512 hash function for @noble/ed25519 v3

@noble/ed25519 v3.0.0 requires explicit SHA-512 hash function setup
before using any cryptographic operations. This fixes the
"hashes.sha512 not set" error.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/usernames.ts

@@ -1,5 +1,11 @@
 import * as ed25519 from '@noble/ed25519';
 
+// Set SHA-512 hash function for ed25519 (required in @noble/ed25519 v3+)
+// Uses built-in WebCrypto API
+ed25519.hashes.sha512Async = async (message: Uint8Array) => {
+  return new Uint8Array(await crypto.subtle.digest('SHA-512', message as BufferSource));
+};
+
 /**
  * Username claim result
  */