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-activator.php

@@ -7,7 +7,8 @@ final class Activator
 {
     public static function activate(): void
     {
-        // Stage 2 will register taxonomy + CPT; flush after they're declared.
+        // S2 registrations use rewrite=false; flush is harmless and
+        // forward-compatible with the S4 URL router.
         flush_rewrite_rules();
     }