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>

plans/commerce-on-sx.md

@@ -21,7 +21,7 @@ reconciliation — all auditable via the event log.
 
 ## Status (rolling)
 
-`bash lib/commerce/conformance.sh` → **54/54** (3 suites: catalog, cart, price)
+`bash lib/commerce/conformance.sh` → **66/66** (4 suites: catalog, cart, price, api) — Phase 1 done
 
 ## Ground rules
 
@@ -58,7 +58,7 @@ lib/commerce/api.sx ── (commerce/add) (commerce/total) (commerce/checkout)
 - [x] `catalog.sx` — product/variant/stock as facts
 - [x] `cart.sx` — line items, add/remove/qty
 - [x] `price.sx` — base pricing relation, subtotal; tax
-- [ ] `api.sx` + tests + scoreboard + conformance.sh
+- [x] `api.sx` + tests + scoreboard + conformance.sh
 
 ## Phase 2 — Promotions (relational)
 - [ ] promo rules: percentage, fixed, bundle, member rate
@@ -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 — `api.sx` (**Phase 1 complete**): session facade
+  `{:ctx :cart}` with `commerce-add`/`-remove`/`-set-qty`/`-total`/`-count`/
+  `-lines`, `commerce-can-add?` catalog validation, `commerce-explain` per-line
+  audit breakdown ({:sku :variant :qty :unit :extended :tax}), and a
+  `commerce-checkout` Phase-3 stub. api suite 12/12; total 66/66.
 - 2026-06-06 — `price.sx`: deterministic `cart-subtotal` (Σ unit×qty, variant
   delta defaults 0) + jurisdiction-relational tax. `taxo` facts indexed by
   (jurisdiction, product-class, customer-class)→bps, queried multidirectionally;