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/scoreboard.md

@@ -18,4 +18,5 @@ _Generated by `lib/commerce/conformance.sh`_
 | attribution | 16 | 0 | 16 |
 | payment | 7 | 0 | 7 |
 | window | 19 | 0 | 19 |
-| **Total** | **228** | **0** | **228** |
+| nettax | 11 | 0 | 11 |
+| **Total** | **239** | **0** | **239** |