commerce: discount-aware (net) tax policy (11 tests) — Phase 5 ext

nettax.sx — alternative to quote.sx's gross-tax default: cart-quote-net taxes
the net (post-discount) base. allocate-discount spreads the basket discount
across lines by extended-price share with a deterministic largest-remainder
pass so per-line shares sum exactly to the discount; each line taxed on its net
at its class rate. Both policies reproducible; pick per jurisdiction.
Total 239/239 across 15 suites.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/commerce/nettax.sx

@@ -0,0 +1,80 @@
+;; lib/commerce/nettax.sx — discount-aware tax (alternative policy).
+;;
+;; price.sx / quote.sx tax the GROSS per-line amounts (discount reduces payable
+;; but not the tax base). This module is the alternative explicit policy: tax the
+;; NET (post-discount) base. The basket-level discount is allocated across lines
+;; in proportion to each line's extended price, with a deterministic
+;; largest-remainder pass so per-line shares sum EXACTLY to the discount; tax is
+;; then charged on each line's net at its class rate.
+;;
+;; Both policies are reproducible from (ctx, cart, ruleset, exclusions); pick the
+;; one the jurisdiction requires. cart-quote-net mirrors cart-quote's shape.
+
+(define ct-sum (fn (xs) (reduce (fn (a x) (+ a x)) 0 xs)))
+
+;; Add 1 to the first `rem` elements (deterministic remainder distribution).
+(define
+  ct-add-rem
+  (fn
+    (xs rem)
+    (cond
+      ((empty? xs) (list))
+      ((> rem 0)
+        (cons
+          (+ (first xs) 1)
+          (ct-add-rem (rest xs) (- rem 1))))
+      (:else xs))))
+
+;; Per-line discount allocation (parallel to cart), summing exactly to
+;; total-discount, proportional to line-extended share.
+(define
+  allocate-discount
+  (fn
+    (cat cart total-discount)
+    (let
+      ((sub (cart-subtotal cat cart)))
+      (if
+        (= sub 0)
+        (map (fn (l) 0) cart)
+        (let
+          ((floors (map (fn (l) (quotient (* total-discount (line-extended cat l)) sub)) cart)))
+          (ct-add-rem floors (- total-discount (ct-sum floors))))))))
+
+;; Tax on one line's net (extended - allocated discount), clamped at 0.
+(define
+  net-line-tax
+  (fn
+    (ctx line alloc)
+    (let
+      ((cat (ctx-catalog ctx)))
+      (let
+        ((net (- (line-extended cat line) alloc)))
+        (apply-bps
+          (if (< net 0) 0 net)
+          (rate-bps
+            (get ctx :tax-rules)
+            (get ctx :jurisdiction)
+            (catalog-class cat (line-sku line))
+            (get ctx :customer)))))))
+
+(define
+  net-tax
+  (fn
+    (ctx cart allocations)
+    (ct-sum
+      (map (fn (line alloc) (net-line-tax ctx line alloc)) cart allocations))))
+
+;; Discount-aware quote: tax computed on the net (post-discount) base.
+(define
+  cart-quote-net
+  (fn
+    (ctx cart ruleset exclusions)
+    (let
+      ((cat (ctx-catalog ctx)))
+      (let
+        ((sub (cart-subtotal cat cart))
+          (disc (best-promo-discount ctx cart ruleset exclusions))
+          (codes (best-promo-codes ctx cart ruleset exclusions)))
+        (let
+          ((tax (net-tax ctx cart (allocate-discount cat cart disc))))
+          {:codes codes :subtotal sub :discount disc :total (+ (- sub disc) tax) :tax tax})))))