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/tests/attribution.sx

@@ -0,0 +1,124 @@
+;; lib/commerce/tests/attribution.sx — line-level discount attribution.
+;; Uses (commerce-test name got expected) provided by conformance.sh.
+
+(define
+  pcat
+  (make-catalog
+    (list
+      (list "widget" 1000 :standard)
+      (list "gizmo" 2000 :standard)
+      (list "book" 800 :zero-rated)
+      (list "tea" 1000 :reduced))
+    (list)
+    (list)))
+
+(define gctx (make-pricing-context pcat (list) :uk :guest))
+(define mctx (make-pricing-context pcat (list) :uk :member))
+
+(define
+  cart
+  (list
+    (list "widget" :none 2)
+    (list "gizmo" :none 1)
+    (list "book" :none 1)
+    (list "tea" :none 6)))
+
+(define
+  ruleset
+  (list
+    (list :percent "TEN" :standard 1000)
+    (list :percent "TWENTY" :standard 2000)
+    (list :bundle "B3T" "tea" 3)
+    (list :fixed "FIVE" 0 500)
+    (list :member "MEM" :standard 1500)))
+
+(define w-line (list "widget" :none 2))
+(define t-line (list "tea" :none 6))
+(define bk-line (list "book" :none 1))
+
+;; --- scope helpers ---
+
+(commerce-test
+  "class-lines-standard"
+  (class-lines gctx cart :standard)
+  (list (list "widget" :none 2) (list "gizmo" :none 1)))
+
+(commerce-test
+  "promo-lines-bundle"
+  (promo-lines gctx cart (list :bundle "B3T" "tea" 3))
+  (list (list "tea" :none 6)))
+
+(commerce-test
+  "promo-lines-fixed-none"
+  (promo-lines gctx cart (list :fixed "FIVE" 0 500))
+  (list))
+
+;; --- forward: which lines does a code touch? ---
+
+(commerce-test
+  "lines-for-ten"
+  (lines-for-code gctx cart ruleset "TEN")
+  (list (list "widget" :none 2) (list "gizmo" :none 1)))
+
+(commerce-test
+  "lines-for-bundle"
+  (lines-for-code gctx cart ruleset "B3T")
+  (list (list "tea" :none 6)))
+
+(commerce-test
+  "lines-for-fixed-empty"
+  (lines-for-code gctx cart ruleset "FIVE")
+  (list))
+(commerce-test
+  "lines-for-mem-guest-empty"
+  (lines-for-code gctx cart ruleset "MEM")
+  (list))
+
+;; --- backward: which codes touch this line? (the showcase) ---
+
+(commerce-test
+  "codes-for-widget-guest"
+  (codes-for-line gctx cart ruleset w-line)
+  (list "TEN" "TWENTY"))
+
+(commerce-test
+  "codes-for-tea"
+  (codes-for-line gctx cart ruleset t-line)
+  (list "B3T"))
+(commerce-test
+  "codes-for-book-none"
+  (codes-for-line gctx cart ruleset bk-line)
+  (list))
+
+;; member sees the member rate too
+(commerce-test
+  "codes-for-widget-member"
+  (codes-for-line mctx cart ruleset w-line)
+  (list "TEN" "TWENTY" "MEM"))
+
+(commerce-test
+  "lines-for-mem-member"
+  (lines-for-code mctx cart ruleset "MEM")
+  (list (list "widget" :none 2) (list "gizmo" :none 1)))
+
+;; --- predicate ---
+
+(commerce-test
+  "touched-yes"
+  (line-touched-by? gctx cart ruleset "TEN" w-line)
+  true)
+(commerce-test
+  "touched-no-wrong-class"
+  (line-touched-by? gctx cart ruleset "B3T" w-line)
+  false)
+(commerce-test
+  "touched-no-guest-mem"
+  (line-touched-by? gctx cart ruleset "MEM" w-line)
+  false)
+
+;; --- order-level (fixed) codes ---
+
+(commerce-test
+  "order-level"
+  (order-level-codes gctx cart ruleset)
+  (list "FIVE"))