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/scoreboard.json

@@ -1,8 +1,9 @@
 {
   "suites": {
-    "catalog": {"pass": 16, "fail": 0}
+    "catalog": {"pass": 16, "fail": 0},
+    "cart": {"pass": 18, "fail": 0}
   },
-  "total_pass": 16,
+  "total_pass": 34,
   "total_fail": 0,
-  "total": 16
+  "total": 34
 }