chore(wpmc S2): defensive defines + PostType::SLUG refs + stale comment

Wrap 4 define() calls with defined() || guards to survive double-load.
Replace hardcoded 'shortcode-town' strings in Taxonomy and FieldGroup
with PostType::SLUG. Update Activator comment to reflect S2 reality.

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

includes/class-taxonomy.php

@@ -14,7 +14,7 @@ final class Taxonomy
 
     public static function do_register(): void
     {
-        register_taxonomy(self::SLUG, ['shortcode-town'], [
+        register_taxonomy(self::SLUG, [PostType::SLUG], [
             'labels'             => [
                 'name'          => __('Города', 'wp-multi-city'),
                 'singular_name' => __('Город', 'wp-multi-city'),