fix: expose the named Builder constructor declared in types

spec/exports_spec.js

@@ -0,0 +1,12 @@
+"use strict";
+
+import * as builderModule from "../src/fxb.js";
+
+describe("builder exports", function () {
+    // 2026-10-11: The named constructor promised by the declarations must be available at runtime.
+    it("should export the same constructor as Builder and default", function () {
+        expect(builderModule.Builder).toBe(builderModule.default);
+        const builder = new builderModule.Builder();
+        expect(builder.build({ root: 'value' })).toEqual('<root>value</root>');
+    });
+});

src/fxb.js

@@ -100,6 +100,9 @@ export default function Builder(options) {
   }
 }
 
+// 2026-10-11: Keep the declared named constructor available alongside the default export.
+export { Builder };
+
 /**
  * Detect XML version from the ?xml declaration at the root of a plain-object input.
  * Checks both attributesGroupName and flat attribute forms.