commerce: public session API + per-line audit + checkout stub (12 tests) — Phase 1 done

api.sx — session facade {:ctx :cart}: commerce-add/remove/set-qty/total/
count/lines, commerce-can-add? catalog validation, commerce-explain per-line
audit breakdown, commerce-checkout Phase-3 stub. Completes Phase 1 (catalog +
cart + deterministic totals). Total 66/66 across 4 suites.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/commerce/api.sx

@@ -0,0 +1,56 @@
+;; lib/commerce/api.sx — public commerce surface.
+;;
+;; A session bundles a pricing context with a cart: {:ctx CTX :cart CART}.
+;; All operations are pure and return a new session. The total and the
+;; per-line breakdown are deterministic functions of (ctx, cart).
+;;
+;; commerce-checkout is a Phase-3 stub — the order lifecycle is a durable
+;; flow that suspends at the SumUp payment boundary.
+
+(define commerce-session (fn (ctx) {:cart empty-cart :ctx ctx}))
+
+(define commerce-ctx (fn (sess) (get sess :ctx)))
+(define commerce-cart (fn (sess) (get sess :cart)))
+(define commerce-lines (fn (sess) (cart-lines (get sess :cart))))
+(define commerce-count (fn (sess) (cart-count (get sess :cart))))
+
+(define
+  commerce-add
+  (fn
+    (sess sku variant qty)
+    (assoc sess :cart (cart-add (get sess :cart) sku variant qty))))
+
+(define
+  commerce-remove
+  (fn
+    (sess sku variant)
+    (assoc sess :cart (cart-remove (get sess :cart) sku variant))))
+
+(define
+  commerce-set-qty
+  (fn
+    (sess sku variant qty)
+    (assoc sess :cart (cart-set-qty (get sess :cart) sku variant qty))))
+
+;; True when the sku exists in the session's catalog snapshot.
+(define
+  commerce-can-add?
+  (fn (sess sku) (catalog-has? (ctx-catalog (get sess :ctx)) sku)))
+
+(define
+  commerce-total
+  (fn (sess) (cart-total (get sess :ctx) (get sess :cart))))
+
+;; Per-line audit breakdown — the "which line contributed what" view.
+(define
+  line-detail
+  (fn (ctx line) (let ((cat (ctx-catalog ctx))) {:sku (line-sku line) :unit (line-unit-price cat (line-sku line) (line-variant line)) :qty (line-qty line) :variant (line-variant line) :extended (line-extended cat line) :tax (line-tax ctx line)})))
+
+(define
+  commerce-explain
+  (fn
+    (sess)
+    (map (fn (l) (line-detail (get sess :ctx) l)) (get sess :cart))))
+
+;; Phase 3 — order lifecycle flow (reserve -> pay -> fulfil) lands here.
+(define commerce-checkout (fn (sess) {:note "order lifecycle flow lands in Phase 3" :phase 3 :status :not-implemented}))