test: add unit tests for frontend top-down auto-reset and backend split logic

shop/lib/checkout-split.ts

@@ -0,0 +1,17 @@
+export interface CartItem {
+  id?: string;
+  billingInterval?: 'one_time' | 'monthly';
+  billingType?: 'purchase' | 'subscription';
+  selections: any;
+  moduleQuantities?: Record<string, number>;
+}
+
+export function splitCart(items: CartItem[]) {
+  const purchaseItems = items.filter(
+    item => item.billingInterval === 'one_time' || item.billingType === 'purchase'
+  );
+  const subscriptionItems = items.filter(
+    item => item.billingInterval === 'monthly' || item.billingType === 'subscription'
+  );
+  return { purchaseItems, subscriptionItems };
+}