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.json

@@ -13,9 +13,10 @@
     "federation": {"pass": 12, "fail": 0},
     "attribution": {"pass": 16, "fail": 0},
     "payment": {"pass": 7, "fail": 0},
-    "window": {"pass": 19, "fail": 0}
+    "window": {"pass": 19, "fail": 0},
+    "nettax": {"pass": 11, "fail": 0}
   },
-  "total_pass": 228,
+  "total_pass": 239,
   "total_fail": 0,
-  "total": 228
+  "total": 239
 }