Storefront API checkout and payment contracts

Storefront checkout and payment flows now have browser-safe, named API contracts.

Storefront API now documents browser-safe guest checkout preparation for
shipping options and checkout summary. Public guest checkout can use a guest
cart token or direct variant items, returns the same shipping option fields as
the hosted storefront, and exposes summary fields as
product_price, shipping_cost, other_income, other_income_name, and
gross_revenue. OpenAPI now also includes named schemas for customer
checkout endpoints and remaining normal-shopping storefront routes such as
categories, variant pricing, cart merge, public order read/update, and
discount-code checks. Storefront API docs now include checkout and payment
rendering guidance based on the hosted success page behavior.