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/conformance.sh

@@ -17,7 +17,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(catalog cart price api promo stack quote ledger order recon federation attribution payment window)
+SUITES=(catalog cart price api promo stack quote ledger order recon federation attribution payment window nettax)
 
 OUT_JSON="lib/commerce/scoreboard.json"
 OUT_MD="lib/commerce/scoreboard.md"
@@ -67,6 +67,7 @@ run_suite() {
 (load "lib/commerce/stack.sx")
 (load "lib/commerce/quote.sx")
 (load "lib/commerce/window.sx")
+(load "lib/commerce/nettax.sx")
 (load "lib/commerce/ledger.sx")
 (load "lib/commerce/order.sx")
 (load "lib/commerce/payment.sx")