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/Shop/Part3.php

@@ -301,7 +301,7 @@ trait Part3 {
             if ($pid <= 0 || $pay_price <= 0) continue;
 
             $rebate_rate = (float) get_post_meta($pid, '_aksrt_rebate_rate', true);
-            if ($rebate_rate <= 0 && class_exists('TermMeta')) {
+            if ($rebate_rate <= 0 && class_exists('AKSRT\Shop\TermMeta')) {
                 $resolved = TermMeta::resolve_inheritance($pid, 'rebate');
                 $rebate_rate = (float) ($resolved ?: 0);
             }