feat(wpmc 1.0.1): wpmc_main_town_slug procedural wrapper

Add wpmc_main_town_slug() wrapper function to wpmc-functions.php, delegating
to Helpers::main_town_slug(). The function was documented in the Hook API
README but was missing from the codebase.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

includes/wpmc-functions.php

@@ -30,3 +30,10 @@ if (!function_exists('wpmc_remove_domain_link')) {
         return Helpers::remove_domain_link($url);
     }
 }
+
+if (!function_exists('wpmc_main_town_slug')) {
+    function wpmc_main_town_slug(): ?string
+    {
+        return Helpers::main_town_slug();
+    }
+}