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>

plans/commerce-on-sx.md

@@ -21,7 +21,7 @@ reconciliation — all auditable via the event log.
 
 ## Status (rolling)
 
-`bash lib/commerce/conformance.sh` → **185/185** (11 suites: catalog, cart, price, api, promo, stack, quote, ledger, order, recon, federation) — **Phases 1-4 done (roadmap complete)**
+`bash lib/commerce/conformance.sh` → **201/201** (12 suites; + attribution) — **roadmap complete; Phase 5 extensions in progress**
 
 ## Ground rules
 
@@ -78,9 +78,9 @@ lib/commerce/api.sx ── (commerce/add) (commerce/total) (commerce/checkout)
 ## Phase 5 — Extensions (backlog; base roadmap complete)
 Thesis-aligned deepenings of the relational/composition showcase. Pick the one
 that unlocks the most tests per effort each iteration.
-- [ ] line-level discount attribution — "which line item triggered this discount?"
-  as a backward miniKanren query (briefing gotcha; promos currently apply at
-  class level). Refactor promo amounts to also yield per-line attribution.
+- [x] line-level discount attribution — "which line item triggered this discount?"
+  as a backward miniKanren query (`attribution.sx`: `promo-toucheso` relation,
+  `lines-for-code`/`codes-for-line` both directions, `order-level-codes` for fixed).
 - [ ] time-windowed promotions — promos gated by a validity window; quote takes a
   datetime, determinism preserved. (quote.sx already documents datetime intent.)
 - [ ] discount-aware tax policy — alternative `cart-quote` computing tax on the
@@ -97,6 +97,14 @@ that unlocks the most tests per effort each iteration.
   Keeps lib/commerce provider-agnostic; enables multi-provider support in the core.
 
 ## Progress log
+- 2026-06-07 — `attribution.sx` (Phase 5 ext): line-level discount attribution —
+  the briefing's marquee "which line item triggered this discount?" query.
+  `promo-lines` is the pure per-promo scope (percent/member → class lines, bundle
+  → sku lines, fixed → order-level/none); `promo-toucheso` relates (code, line)
+  for applying promos, run forward (`lines-for-code`) and backward
+  (`codes-for-line`). `order-level-codes` lists applying fixed promos; predicate
+  `line-touched-by?`. Additive — promo.sx amounts unchanged. attribution suite
+  16/16; total 201/201 (12 suites).
 - 2026-06-07 — `recon.sx` + `federation.sx` (**Phase 4 complete — roadmap done**).
   `recon.sx`: reconciliation as relational queries over the ledger. Per-order
   summary tuples (id total paid refunded net status); `recon-statuso`/`neto`/