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/scoreboard.json

@@ -10,9 +10,10 @@
     "ledger": {"pass": 20, "fail": 0},
     "order": {"pass": 21, "fail": 0},
     "recon": {"pass": 20, "fail": 0},
-    "federation": {"pass": 12, "fail": 0}
+    "federation": {"pass": 12, "fail": 0},
+    "attribution": {"pass": 16, "fail": 0}
   },
-  "total_pass": 185,
+  "total_pass": 201,
   "total_fail": 0,
-  "total": 185
+  "total": 201
 }