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>

lib/commerce/tests/cart.sx

@@ -0,0 +1,103 @@
+;; lib/commerce/tests/cart.sx — cart structure + line operations.
+;; Uses (commerce-test name got expected) provided by conformance.sh.
+
+;; --- add ---
+
+(commerce-test
+  "add-to-empty"
+  (cart-add empty-cart "widget" :small 2)
+  (list (list "widget" :small 2)))
+
+(commerce-test
+  "add-merges-same-line"
+  (cart-add
+    (cart-add empty-cart "widget" :small 2)
+    "widget"
+    :small 3)
+  (list (list "widget" :small 5)))
+
+(commerce-test
+  "add-different-variant-separate"
+  (cart-add
+    (cart-add empty-cart "widget" :small 2)
+    "widget"
+    :large 1)
+  (list (list "widget" :small 2) (list "widget" :large 1)))
+
+(commerce-test
+  "add-different-sku-separate"
+  (cart-add
+    (cart-add empty-cart "widget" :small 2)
+    "gadget"
+    :std 1)
+  (list (list "widget" :small 2) (list "gadget" :std 1)))
+
+(commerce-test
+  "add-preserves-order"
+  (cart-skus
+    (cart-add
+      (cart-add (cart-add empty-cart "a" :v 1) "b" :v 1)
+      "c"
+      :v 1))
+  (list "a" "b" "c"))
+
+;; --- qty queries ---
+
+(define
+  c2
+  (cart-add
+    (cart-add empty-cart "widget" :small 2)
+    "gadget"
+    :std 4))
+
+(commerce-test "cart-qty-found" (cart-qty c2 "widget" :small) 2)
+(commerce-test "cart-qty-missing" (cart-qty c2 "widget" :large) 0)
+(commerce-test "cart-count" (cart-count c2) 6)
+(commerce-test "cart-empty-yes" (cart-empty? empty-cart) true)
+(commerce-test "cart-empty-no" (cart-empty? c2) false)
+
+;; --- set-qty ---
+
+(commerce-test
+  "set-qty-existing"
+  (cart-set-qty c2 "widget" :small 10)
+  (list (list "widget" :small 10) (list "gadget" :std 4)))
+
+(commerce-test
+  "set-qty-new-line"
+  (cart-set-qty empty-cart "book" :std 3)
+  (list (list "book" :std 3)))
+
+(commerce-test
+  "set-qty-zero-removes"
+  (cart-set-qty c2 "widget" :small 0)
+  (list (list "gadget" :std 4)))
+
+;; --- remove ---
+
+(commerce-test
+  "remove-line"
+  (cart-remove c2 "gadget" :std)
+  (list (list "widget" :small 2)))
+
+(commerce-test
+  "remove-missing-noop"
+  (cart-remove c2 "nope" :std)
+  (list (list "widget" :small 2) (list "gadget" :std 4)))
+
+;; --- relational view ---
+
+(commerce-test
+  "cart-lineo-forward"
+  (run* q (cart-lineo c2 "gadget" :std q))
+  (list 4))
+
+(commerce-test
+  "cart-lineo-sku-by-qty-backward"
+  (run* sk (fresh (v) (cart-lineo c2 sk v 4)))
+  (list "gadget"))
+
+(commerce-test
+  "cart-lineo-all-skus"
+  (run* sk (fresh (v q) (cart-lineo c2 sk v q)))
+  (list "widget" "gadget"))