docs(checkout): CHECKOUT-9857 Document storefront checkout shipping comparison fields

[bc-mackxu]

…

CHECKOUT-9857

What changed?

Updates the Storefront Checkouts spec (reference/checkouts.sf.yml) for the Get a Checkout response.

• checkout_Full: clarify shippingCostBeforeDiscount (total shipping before discounts) and document comparisonShippingCost (total shipping after automatic promotions).
• consignment_Full: add shippingCostBeforeDiscount and comparisonShippingCost with the same meanings per consignment.
• CheckoutResp example: include the new fields.

Release notes draft

• The Storefront Checkout API response now documents shipping comparison fields so you can show shoppers the right “original” vs. promotional shipping amounts. Checkout and each consignment include shipping cost before discount and shipping cost after automatic promotions, which makes it easier to display fair pricing and explain savings at checkout.

Anything else?

ping @bigcommerce/team-checkout