commerce: promo rules (percent/fixed/bundle/member) as relations (17 tests)

promo.sx — four promo types as tagged tuples; per-promo discount is pure
integer arithmetic, but enumeration is relational: promo-discounto and
promo-applieso run forward ("which codes apply, for how much?") and backward
("which code yields this discount?"). project grounds the membero-bound promo.
applicable-promos / promo-amount-for deterministic helpers. Total 83/83.

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

lib/commerce/scoreboard.json

@@ -3,9 +3,10 @@
     "catalog": {"pass": 16, "fail": 0},
     "cart": {"pass": 18, "fail": 0},
     "price": {"pass": 20, "fail": 0},
-    "api": {"pass": 12, "fail": 0}
+    "api": {"pass": 12, "fail": 0},
+    "promo": {"pass": 17, "fail": 0}
   },
-  "total_pass": 66,
+  "total_pass": 83,
   "total_fail": 0,
-  "total": 66
+  "total": 83
 }