commerce: deterministic subtotal + jurisdiction-relational tax (20 tests)

price.sx — cart-subtotal (unit price = base + variant delta, default 0),
taxo facts indexed by (jurisdiction, product-class, customer-class) -> bps
queried both directions, apply-bps half-up integer rounding, cart-total
returning {:subtotal :discounts :tax :total} reproducible from
(context, cart). Total 54/54.

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

lib/commerce/scoreboard.md

@@ -6,4 +6,5 @@ _Generated by `lib/commerce/conformance.sh`_
 |-------|-----:|-----:|------:|
 | catalog | 16 | 0 | 16 |
 | cart | 18 | 0 | 18 |
-| **Total** | **34** | **0** | **34** |
+| price | 20 | 0 | 20 |
+| **Total** | **54** | **0** | **54** |