BREAKING CHANGES:
- Remove storeId and productType from checkout params (derived from productId)
- Remove CheckoutSessionProductType enum
- IssueSessionTokenParams.storeId now optional (provide storeId or productId)

Added:
- IssueSessionTokenParams.productId for store derivation from product
- GraphQL type difference warnings in docs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>