Cart Total Calculator

A cart contains items with a `price` and `quantity`. Build a summary that calculates subtotal, discount, tax, and final total.

1. `calculateSubtotal(items)` should add `price * quantity` for every item.
2. `calculateDiscount(subtotal, discountPercent)` should return the discount amount.
3. `calculateTax(amountAfterDiscount, taxPercent)` should return the tax amount after the discount.
4. `createCartSummary(items, discountPercent, taxPercent)` should return an object with `subtotal`, `discount`, `tax`, and `total`.