feat(pay): guest cash purchase with signed cookie unlock (zibll-style)

Allow pay_no_logged_in cash content orders (user_id=0), set aksrt_pay_* cookies
on paid, accept cookie in access state, and expose guest CTA on paywall/box.

inc/classes/class-aksrt-order.php

@@ -48,7 +48,8 @@ class AKSRT_Order {
     }
 
     public function create(int $user_id, string $product_type, int $product_id, float $amount, string $currency = 'CNY'): string {
-        if ($user_id <= 0 || $product_id < 0 || $amount < 0) {
+        // Guest content orders may use user_id=0 (Zibll pay_no_logged_in).
+        if ($user_id < 0 || $product_id < 0 || $amount < 0) {
             return '';
         }