commerce: stacking precedence + best-price selection + backward query (16 tests) — Phase 2 done

stack.sx — precedence as a separate selection layer, not in the rules.
Exclusivity = unordered code pairs; valid-stackings enumerates every legal
subset of applicable promos; best-stacking deterministically picks max total
discount (stable on ties); stacking-by-totalo answers "which legal stacking
yields total D?" backward. Member vs guest falls out of applicable-promos.
Completes Phase 2. Total 99/99 across 6 suites.

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

plans/commerce-on-sx.md

@@ -21,7 +21,7 @@ reconciliation — all auditable via the event log.
 
 ## Status (rolling)
 
-`bash lib/commerce/conformance.sh` → **83/83** (5 suites: catalog, cart, price, api, promo) — Phase 1 done, Phase 2 in progress
+`bash lib/commerce/conformance.sh` → **99/99** (6 suites: catalog, cart, price, api, promo, stack) — Phases 1-2 done
 
 ## Ground rules
 
@@ -62,8 +62,8 @@ lib/commerce/api.sx ── (commerce/add) (commerce/total) (commerce/checkout)
 
 ## Phase 2 — Promotions (relational)
 - [x] promo rules: percentage, fixed, bundle, member rate
-- [ ] explicit stacking precedence; "best price" backward query
-- [ ] tests: stacking order, mutually-exclusive promos, member vs guest
+- [x] explicit stacking precedence; "best price" backward query
+- [x] tests: stacking order, mutually-exclusive promos, member vs guest
 
 ## Phase 3 — Order lifecycle (flow + store)
 - [ ] order flow: reserve stock → await payment → fulfil
@@ -76,6 +76,14 @@ lib/commerce/api.sx ── (commerce/add) (commerce/total) (commerce/checkout)
 - [ ] tests: webhook replay, partial refund, double-charge guard
 
 ## Progress log
+- 2026-06-07 — `stack.sx` (**Phase 2 complete**): stacking precedence as a
+  separate selection layer (precedence NOT in the rules, per the miniKanren
+  design rule). Exclusivity = unordered code pairs; `valid-stackings` enumerates
+  every legal subset of applicable promos (powerset ∖ excluded combos);
+  `best-stacking` is the deterministic max-total-discount selection (stable on
+  ties). `stacking-by-totalo` is the best-price backward query ("which legal
+  stacking yields total D?"). Member vs guest falls out of applicable-promos.
+  stack suite 16/16; total 99/99.
 - 2026-06-07 — `promo.sx` (Phase 2 piece 1): four promo types as tagged tuples
   `(:percent code class bps)`/`(:fixed code threshold amount)`/`(:bundle code sku
   n)`/`(:member code class bps)`. Per-promo discount is pure integer arithmetic;