feat(wpmc S2): FieldGroup::register() attaches acf/init hook

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

tests/FieldGroupTest.php

@@ -0,0 +1,19 @@
+<?php
+declare(strict_types=1);
+
+namespace WPMultiCity\Tests;
+
+use WPMultiCity\FieldGroup;
+
+final class FieldGroupTest extends TestCase
+{
+    public function test_register_attaches_acf_init_action(): void
+    {
+        FieldGroup::register();
+
+        self::assertNotFalse(
+            has_action('acf/init', [FieldGroup::class, 'do_register']),
+            'FieldGroup::register() must hook acf/init'
+        );
+    }
+}