Preserve explicit model and provider architecture (#30)

frontend/packages/sxchnge_auth/lib/src/dto/results.dart

@@ -0,0 +1,11 @@
+/// Stub response contracts, replaced by generated transport types at integration.
+class AuthDTO {
+  const AuthDTO({required this.userId, required this.simulated});
+  final String userId;
+  final bool simulated;
+}
+
+class VerificationDTO {
+  const VerificationDTO({required this.simulated});
+  final bool simulated;
+}