test(wpmc S2): lock ACF-missing guard for FieldGroup

Add patchwork.json to enable function_exists shimming via Patchwork
(required so Brain Monkey can intercept the PHP internal).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

patchwork.json

@@ -0,0 +1,3 @@
+{
+  "redefinable-internals": ["function_exists"]
+}

tests/FieldGroupTest.php

@@ -16,4 +16,14 @@ final class FieldGroupTest extends TestCase
             'FieldGroup::register() must hook acf/init'
         );
     }
+
+    public function test_do_register_is_noop_when_acf_missing(): void
+    {
+        \Brain\Monkey\Functions\when('function_exists')->justReturn(false);
+        \Brain\Monkey\Functions\expect('acf_add_local_field_group')->never();
+
+        FieldGroup::do_register();
+
+        self::assertTrue(true, 'do_register must not call acf_add_local_field_group when ACF is missing');
+    }
 }