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

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

includes/class-field-group.php

@@ -0,0 +1,20 @@
+<?php
+declare(strict_types=1);
+
+namespace WPMultiCity;
+
+final class FieldGroup
+{
+    public const GROUP_KEY    = 'group_wpmc_shortcode_rows';
+    public const REPEATER_KEY = 'field_wpmc_list_shortcode';
+
+    public static function register(): void
+    {
+        add_action('acf/init', [self::class, 'do_register']);
+    }
+
+    public static function do_register(): void
+    {
+        // Implementation filled in Tasks 13 and 14.
+    }
+}

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'
+        );
+    }
+}