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/CartService.php

@@ -159,7 +159,7 @@ class CartService {
             $guest = get_post_meta($product_id, '_aksrt_guest_buy', true);
             if ($guest === '0' || $guest === '') {
                 // Fall back to inheritance chain via TermMeta
-                if (class_exists('TermMeta')) {
+                if (class_exists('AKSRT\Shop\TermMeta')) {
                     $resolved = TermMeta::resolve_inheritance($product_id, 'guest_buy');
                     if (empty($resolved)) {
                         return 'login_required';

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);
             }

src/AKSRT/Shop/ProductReview.php

@@ -275,7 +275,7 @@ class ProductReview {
             $order_number = (string) ($order['order_number'] ?? '');
             if ($user_id <= 0 || $order_number === '') continue;
 
-            $items = class_exists('Shop')
+            $items = class_exists('AKSRT\\Shop\\Shop')
                 ? Shop::get_instance()->get_order_items($order_number)
                 : [];
 

src/AKSRT/Shop/ProductVariantsMetaBox.php

@@ -41,7 +41,7 @@ class ProductVariantsMetaBox {
         wp_nonce_field('aksrt_product_variants_save', 'aksrt_product_variants_nonce');
 
         $variants = [];
-        if (class_exists('Shop')) {
+        if (class_exists('AKSRT\\Shop\\Shop')) {
             $variants = Shop::get_instance()->get_variants((int) $post->ID);
         }
 
@@ -138,7 +138,7 @@ class ProductVariantsMetaBox {
         if ($post->post_type !== 'aksrt_product' || !current_user_can('edit_post', $post_id)) {
             return;
         }
-        if (!class_exists('Shop')) {
+        if (!class_exists('AKSRT\\Shop\\Shop')) {
             return;
         }