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/conformance.sh

@@ -17,7 +17,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(catalog cart price)
+SUITES=(catalog cart price api)
 
 OUT_JSON="lib/commerce/scoreboard.json"
 OUT_MD="lib/commerce/scoreboard.md"
@@ -46,6 +46,7 @@ run_suite() {
 (load "lib/commerce/catalog.sx")
 (load "lib/commerce/cart.sx")
 (load "lib/commerce/price.sx")
+(load "lib/commerce/api.sx")
 (epoch 2)
 (eval "(define ct-pass 0)")
 (eval "(define ct-fail 0)")