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.

templates/parts/download/download-hero.php

@@ -11,7 +11,7 @@ $page_title = get_the_title() ?: __('下载中心', 'aksrt');
 $page_desc  = get_the_excerpt() ?: __('浏览和下载本站提供的各类资源。', 'aksrt');
 $current_filter = isset($_GET['resource_type']) ? sanitize_key((string) $_GET['resource_type']) : '';
 $resource_type_labels = function_exists('aksrt_get_pay_types') ? aksrt_get_pay_types() : [];
-unset($resource_type_labels[AKSRT_Domain_Model::PAY_TYPE_READING]);
+unset($resource_type_labels[\AKSRT\Core\DomainModel::PAY_TYPE_READING]);
 
 $resource_count_query = new WP_Query([
     'post_type'      => 'post',
@@ -20,7 +20,7 @@ $resource_count_query = new WP_Query([
     'fields'         => 'ids',
     'meta_query'     => [
         [
-            'key'   => AKSRT_Domain_Model::RESOURCE_MARKER_KEY,
+            'key'   => \AKSRT\Core\DomainModel::RESOURCE_MARKER_KEY,
             'value' => '1',
         ],
     ],