feat(wpmc S2): Taxonomy::register() attaches init hook

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

includes/class-taxonomy.php

@@ -0,0 +1,19 @@
+<?php
+declare(strict_types=1);
+
+namespace WPMultiCity;
+
+final class Taxonomy
+{
+    public const SLUG = 'town';
+
+    public static function register(): void
+    {
+        add_action('init', [self::class, 'do_register']);
+    }
+
+    public static function do_register(): void
+    {
+        // Implementation filled in Task 5.
+    }
+}