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/conformance.sh

@@ -17,7 +17,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(catalog cart price api)
+SUITES=(catalog cart price api promo)
 
 OUT_JSON="lib/commerce/scoreboard.json"
 OUT_MD="lib/commerce/scoreboard.md"
@@ -47,6 +47,7 @@ run_suite() {
 (load "lib/commerce/cart.sx")
 (load "lib/commerce/price.sx")
 (load "lib/commerce/api.sx")
+(load "lib/commerce/promo.sx")
 (epoch 2)
 (eval "(define ct-pass 0)")
 (eval "(define ct-fail 0)")