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.md

@@ -15,4 +15,5 @@ _Generated by `lib/commerce/conformance.sh`_
 | order | 21 | 0 | 21 |
 | recon | 20 | 0 | 20 |
 | federation | 12 | 0 | 12 |
-| **Total** | **185** | **0** | **185** |
+| attribution | 16 | 0 | 16 |
+| **Total** | **201** | **0** | **201** |