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/modules/user-hub.php

@@ -0,0 +1,15 @@
+<?php
+/**
+ * AKSRT User system module hub (procedural).
+ *
+ * Loads user-parts/*; keeps historical path for bootstrap.
+ *
+ * @package AKSRT
+ */
+
+defined('ABSPATH') || exit;
+
+require_once __DIR__ . '/user-parts/template-registry.php';
+require_once __DIR__ . '/user-parts/urls.php';
+require_once __DIR__ . '/user-parts/routing.php';
+require_once __DIR__ . '/user-parts/account-ajax.php';