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/docs/doc-chapter-nav.php

@@ -3,7 +3,7 @@
  * Document Chapter Navigation
  *
  * Shows previous/next chapter navigation at the bottom of a document page.
- * Uses AKSRT_Docs_Presenter for chapter resolution.
+ * Uses \AKSRT\Core\DocsPresenter for chapter resolution.
  *
  * @package AKSRT
  */
@@ -12,7 +12,7 @@ defined('ABSPATH') || exit;
 
 $current_id = get_the_ID();
 
-$presenter = class_exists('AKSRT_Docs_Presenter') ? AKSRT_Docs_Presenter::get_instance() : null;
+$presenter = class_exists('\AKSRT\Core\DocsPresenter') ? \AKSRT\Core\DocsPresenter::get_instance() : null;
 
 if ($presenter) {
     $nav = $presenter->get_chapter_nav($current_id);