commerce: line-level discount attribution (16 tests) — Phase 5 ext

attribution.sx — the briefing's marquee "which line item triggered this
discount?" backward query. promo-lines gives each promo's pure scope
(percent/member -> class lines, bundle -> sku lines, fixed -> order-level);
promo-toucheso relates (code, line) for applying promos, run forward
(lines-for-code) and backward (codes-for-line). Additive; promo amounts
unchanged. Total 201/201 across 12 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 stack quote ledger order recon federation)
+SUITES=(catalog cart price api promo stack quote ledger order recon federation attribution)
 
 OUT_JSON="lib/commerce/scoreboard.json"
 OUT_MD="lib/commerce/scoreboard.md"
@@ -70,6 +70,7 @@ run_suite() {
 (load "lib/commerce/order.sx")
 (load "lib/commerce/recon.sx")
 (load "lib/commerce/federation.sx")
+(load "lib/commerce/attribution.sx")
 (epoch 2)
 (eval "(define ct-pass 0)")
 (eval "(define ct-fail 0)")