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>

lib/commerce/conformance.sh

@@ -17,7 +17,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(catalog cart price api promo)
+SUITES=(catalog cart price api promo stack)
 
 OUT_JSON="lib/commerce/scoreboard.json"
 OUT_MD="lib/commerce/scoreboard.md"
@@ -48,6 +48,7 @@ run_suite() {
 (load "lib/commerce/price.sx")
 (load "lib/commerce/api.sx")
 (load "lib/commerce/promo.sx")
+(load "lib/commerce/stack.sx")
 (epoch 2)
 (eval "(define ct-pass 0)")
 (eval "(define ct-fail 0)")