fix(ns): class_exists() must use FQCN after PSR-4

Bare class_exists('Balance') looks up global Balance, always false
under AKSRT\* namespaces → REST 403 on balance exchange and friends.
Rewrite string checks to AKSRT\{Domain}\{Class}.

src/AKSRT/Settings/FacadeBulk.php

@@ -275,7 +275,7 @@ trait FacadeBulk {
      */
     public static function get_post_page_settings(): array {
         // Canonical assembly lives in the post-display helper (related style/source split, aliases).
-        if (class_exists('PostDisplay')) {
+        if (class_exists('AKSRT\\Settings\\PostDisplay')) {
             return PostDisplay::get_post_page_settings();
         }