cart.sx — cart as an ordered list of (sku variant qty) lines. Pure
operations: cart-add (merge-or-append), cart-set-qty (0 removes),
cart-remove, with cart-qty/count/skus/empty? accessors. cart-lineo
exposes lines relationally via membero. Total 34/34.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
