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.

src/AKSRT/Payment/GatewayInterface.php

@@ -0,0 +1,72 @@
+<?php
+/**
+ * @package AKSRT
+ */
+
+namespace AKSRT\Payment;
+
+defined('ABSPATH') || exit;
+
+/**
+ * AKSRT Payment Gateway — 支付网关适配器层
+ *
+ * 定义支付网关统一接口和基础实现。
+ * 具体网关（支付宝、微信支付等）通过子类实现。
+ *
+ * @package AKSRT
+ */
+
+defined('ABSPATH') || exit;
+
+/**
+ * 支付网关统一接口
+ */
+interface GatewayInterface {
+    /**
+     * 创建支付单
+     *
+     * @param array $order 订单数据
+     * @return array{success: bool, payment_url?: string, payment_data?: array, message?: string}
+     */
+    public function create_payment(array $order): array;
+
+    /**
+     * 验证回调
+     *
+     * @param \WP_REST_Request $request 回调请求
+     * @return array{success: bool, order_number?: string, amount?: float, transaction_id?: string, message?: string}
+     */
+    public function verify_callback(\WP_REST_Request $request): array;
+
+    /**
+     * 查询支付状态
+     *
+     * @param string $order_number 订单号
+     * @return array{success: bool, status?: string, transaction_id?: string, message?: string}
+     */
+    public function query_status(string $order_number): array;
+
+    /**
+     * 退款
+     *
+     * @param string $order_number 订单号
+     * @param float  $amount       退款金额
+     * @return array{success: bool, refund_id?: string, message?: string}
+     */
+    public function refund(string $order_number, float $amount): array;
+
+    /**
+     * 获取网关显示名称
+     */
+    public function get_name(): string;
+
+    /**
+     * 获取网关标识
+     */
+    public function get_id(): string;
+
+    /**
+     * 网关是否已配置
+     */
+    public function is_configured(): bool;
+}