feat(wpmc S4): OptionsPage scaffold + main_town_slug filter wiring

includes/class-options-page.php

@@ -0,0 +1,21 @@
+<?php
+declare(strict_types=1);
+
+namespace WPMultiCity;
+
+final class OptionsPage
+{
+    public const PAGE_SLUG = 'wp-multi-city';
+    public const GROUP_KEY = 'group_wpmc_routing';
+
+    public static function register(): void
+    {
+        \add_action('acf/init', [self::class, 'do_register']);
+        \add_filter('wpmc_main_town_slug', [Helpers::class, 'main_town_slug'], 5);
+    }
+
+    public static function do_register(): void
+    {
+        // Implementation in Task 8.
+    }
+}