feat(headerspec): add UTF-8 encoding/decoding support

* Implement round-trip encoding/decoding for UTF-8 values in header functions.
* Update encodeHeaderValue and decodeHeaderValue to use base64 utility functions.
* Add tests for UTF-8 header value handling.
* Update Vite config to externalize base64 utility.

resolvespec-js/vite.config.ts

@@ -14,7 +14,7 @@ export default defineConfig({
             fileName: (format) => `index.${format === 'es' ? 'js' : 'cjs'}`,
         },
         rollupOptions: {
-            external: ['uuid', 'semver'],
+            external: ['uuid', 'semver', '@warkypublic/artemis-kit/base64'],
         },
     },
 });