commerce: cart line items + add/remove/set-qty + relational view (18 tests)

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>

plans/commerce-on-sx.md

@@ -21,7 +21,7 @@ reconciliation — all auditable via the event log.
 
 ## Status (rolling)
 
-`bash lib/commerce/conformance.sh` → **16/16** (1 suite: catalog)
+`bash lib/commerce/conformance.sh` → **34/34** (2 suites: catalog, cart)
 
 ## Ground rules
 
@@ -56,7 +56,7 @@ lib/commerce/api.sx ── (commerce/add) (commerce/total) (commerce/checkout)
 
 ## Phase 1 — Catalog + cart + deterministic totals
 - [x] `catalog.sx` — product/variant/stock as facts
-- [ ] `cart.sx` — line items, add/remove/qty
+- [x] `cart.sx` — line items, add/remove/qty
 - [ ] `price.sx` — base pricing relation, subtotal; tax
 - [ ] `api.sx` + tests + scoreboard + conformance.sh
 
@@ -76,6 +76,11 @@ lib/commerce/api.sx ── (commerce/add) (commerce/total) (commerce/checkout)
 - [ ] tests: webhook replay, partial refund, double-charge guard
 
 ## Progress log
+- 2026-06-06 — `cart.sx`: cart as an ordered list of (sku variant qty) lines.
+  Pure ops `cart-add` (merges same line / appends), `cart-set-qty` (0 removes),
+  `cart-remove`, plus `cart-qty`/`cart-count`/`cart-skus`/`cart-empty?`.
+  `cart-lineo` is the relational view (membero over the cart) — forward and
+  backward. cart suite 18/18; total 34/34.
 - 2026-06-06 — `catalog.sx`: catalog snapshot (products/variants/stock as fact
   tuples) + multidirectional accessor relations (`producto`/`varianto`/`stocko`,
   derived `priceo`/`classo`/`unit-priceo`) + deterministic `catalog-price`/