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/scoreboard.json

@@ -2,9 +2,10 @@
   "suites": {
     "catalog": {"pass": 16, "fail": 0},
     "cart": {"pass": 18, "fail": 0},
-    "price": {"pass": 20, "fail": 0}
+    "price": {"pass": 20, "fail": 0},
+    "api": {"pass": 12, "fail": 0}
   },
-  "total_pass": 54,
+  "total_pass": 66,
   "total_fail": 0,
-  "total": 54
+  "total": 66
 }