feat(wpmc S4): Router scaffold + query_vars filter for town

includes/class-router.php

@@ -0,0 +1,24 @@
+<?php
+declare(strict_types=1);
+
+namespace WPMultiCity;
+
+final class Router
+{
+    public static function register(): void
+    {
+        \add_action('wp', [self::class, 'route'], 9999);
+        \add_filter('query_vars', [self::class, 'add_town_query_var']);
+    }
+
+    public static function add_town_query_var(array $vars): array
+    {
+        $vars[] = 'town';
+        return $vars;
+    }
+
+    public static function route(): void
+    {
+        // Implementation in Tasks 10-13.
+    }
+}