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.md

@@ -8,4 +8,5 @@ _Generated by `lib/commerce/conformance.sh`_
 | cart | 18 | 0 | 18 |
 | price | 20 | 0 | 20 |
 | api | 12 | 0 | 12 |
-| **Total** | **66** | **0** | **66** |
+| promo | 17 | 0 | 17 |
+| **Total** | **83** | **0** | **83** |