refactor(autoload): migrate theme classes to Composer PSR-4

Move 288 types from global AKSRT_* + inc/classes manual requires into
namespace AKSRT\{Domain} under src/AKSRT/, wire composer.json psr-4,
and slim bootstrap to helpers/modules/hooks only. Split multi-class
widget/gateway files, update admin contract audit paths, and document
the layout in ADR/README.

inc/helpers/aksrt-coupon-order.php

@@ -15,11 +15,11 @@ if (!function_exists('aksrt_coupon_usage_exists')) {
      * Whether a coupon_usage row already exists for this order.
      */
     function aksrt_coupon_usage_exists(string $order_number): bool {
-        if ($order_number === '' || !class_exists('AKSRT_DB_Migrator')) {
+        if ($order_number === '' || !class_exists('\AKSRT\Core\DBMigrator')) {
             return false;
         }
         global $wpdb;
-        $table = AKSRT_DB_Migrator::table('coupon_usage');
+        $table = \AKSRT\Core\DBMigrator::table('coupon_usage');
         if ($table === '') {
             return false;
         }
@@ -39,7 +39,7 @@ if (!function_exists('aksrt_burn_order_coupon')) {
      * @return bool True when a burn was attempted (or already burned).
      */
     function aksrt_burn_order_coupon(string $order_number, array $meta, int $buyer_id = 0): bool {
-        if ($order_number === '' || !class_exists('AKSRT_Coupon')) {
+        if ($order_number === '' || !class_exists('\AKSRT\Payment\Coupon')) {
             return false;
         }
         if (aksrt_coupon_usage_exists($order_number)) {
@@ -53,7 +53,7 @@ if (!function_exists('aksrt_burn_order_coupon')) {
             return false;
         }
 
-        $coupon_api = AKSRT_Coupon::get_instance();
+        $coupon_api = \AKSRT\Payment\Coupon::get_instance();
         $coupon_api->use_coupon($coupon_code);
 
         // Prefer snapshot id: post-increment validate may fail on usage_limit edge.