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/scoreboard.json

@@ -4,9 +4,10 @@
     "cart": {"pass": 18, "fail": 0},
     "price": {"pass": 20, "fail": 0},
     "api": {"pass": 12, "fail": 0},
-    "promo": {"pass": 17, "fail": 0}
+    "promo": {"pass": 17, "fail": 0},
+    "stack": {"pass": 16, "fail": 0}
   },
-  "total_pass": 83,
+  "total_pass": 99,
   "total_fail": 0,
-  "total": 83
+  "total": 99
 }