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/Discount.php

@@ -0,0 +1,167 @@
+<?php
+/**
+ * @package AKSRT
+ */
+
+namespace AKSRT\Payment;
+
+defined('ABSPATH') || exit;
+
+class Discount {
+
+    private static $instance = null;
+
+    public static function get_instance(): self {
+        if (null === self::$instance) {
+            self::$instance = new self();
+        }
+        return self::$instance;
+    }
+
+    /**
+     * Get all active discount campaigns for a product.
+     *
+     * @param int $product_id
+     * @return array<int, array> Sorted by priority desc.
+     */
+    public function get_active_discounts(int $product_id): array {
+        $terms = wp_get_object_terms($product_id, 'shop_discount', ['fields' => 'all']);
+        if (is_wp_error($terms) || empty($terms)) {
+            return [];
+        }
+
+        $now = current_time('timestamp');
+        $active = [];
+
+        foreach ($terms as $term) {
+            $meta = \AKSRT\Shop\TermMeta::get_discount_meta($term->term_id);
+
+            // Time limit check
+            if (!empty($meta['start']) && strtotime($meta['start']) > $now) {
+                continue;
+            }
+            if (!empty($meta['end']) && strtotime($meta['end']) < $now) {
+                continue;
+            }
+
+            $meta['term_id'] = $term->term_id;
+            $meta['name'] = $term->name;
+            $active[] = $meta;
+        }
+
+        // Sort by priority desc
+        usort($active, function ($a, $b) {
+            return ((int) $b['priority']) - ((int) $a['priority']);
+        });
+
+        return $active;
+    }
+
+    /**
+     * Calculate discount for a single line item.
+     *
+     * @param float $unit_price  Unit price
+     * @param int   $quantity    Quantity
+     * @param int   $product_id  Product ID
+     * @param int   $user_id     User ID (for user_limit checks)
+     * @return array{
+     *   original: float,
+     *   discount_amount: float,
+     *   final_price: float,
+     *   gifts: array,
+     *   applied: array<int, array>
+     * }
+     */
+    public function calculate(float $unit_price, int $quantity, int $product_id, int $user_id = 0): array {
+        $original = $unit_price * $quantity;
+        $discount_amount = 0.0;
+        $gifts = [];
+        $applied = [];
+
+        $discounts = $this->get_active_discounts($product_id);
+
+        foreach ($discounts as $d) {
+            // User limit check
+            if (!empty($d['user_limit']) && $user_id > 0) {
+                if (!$this->check_user_limit($d['user_limit'], $user_id)) {
+                    continue;
+                }
+            }
+
+            // Price limit check
+            if ((float) $d['price_limit'] > 0 && $original < (float) $d['price_limit']) {
+                continue;
+            }
+
+            $type = $d['type'] ?? 'reduction';
+            $value = (float) ($d['value'] ?? 0);
+
+            switch ($type) {
+                case 'reduction':
+                    // 立减
+                    $discount_amount += min($value * $quantity, $original - $discount_amount);
+                    break;
+
+                case 'discount':
+                    // 打折 (value = 折扣系数 e.g. 8.5 = 85折)
+                    if ($value > 0 && $value < 10) {
+                        $rate = $value / 10.0;
+                        $save = ($original - $discount_amount) * (1 - $rate);
+                        $discount_amount += $save;
+                    }
+                    break;
+
+                case 'gift':
+                    // 买赠 — collect gift config, no price reduction
+                    if (!empty($d['gift_config'])) {
+                        $gift_data = json_decode($d['gift_config'], true);
+                        if (is_array($gift_data)) {
+                            $gifts[] = $gift_data;
+                        }
+                    }
+                    break;
+            }
+
+            $applied[] = [
+                'term_id' => $d['term_id'],
+                'name'    => $d['name'] ?? '',
+                'type'    => $type,
+                'value'   => $value,
+            ];
+        }
+
+        $discount_amount = min($discount_amount, $original);
+
+        return [
+            'original'        => $original,
+            'discount_amount' => round($discount_amount, 2),
+            'final_price'     => round($original - $discount_amount, 2),
+            'gifts'           => $gifts,
+            'applied'         => $applied,
+        ];
+    }
+
+    /**
+     * Check if a user meets the user_limit requirement.
+     *
+     * @param string $limit  'vip' / 'vip_2' / 'auth'
+     * @param int    $user_id
+     * @return bool
+     */
+    private function check_user_limit(string $limit, int $user_id): bool {
+        if (empty($limit)) {
+            return true;
+        }
+
+        switch ($limit) {
+            case 'vip':
+                return function_exists('aksrt_is_vip') ? aksrt_is_vip($user_id) : false;
+            case 'vip_2':
+                return function_exists('aksrt_is_vip2') ? aksrt_is_vip2($user_id) : false;
+            case 'auth':
+                return function_exists('aksrt_is_verified') ? aksrt_is_verified($user_id) : false;
+            default:
+                return true;
+        }
+    }
+}