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>

plans/commerce-on-sx.md

@@ -21,7 +21,7 @@ reconciliation — all auditable via the event log.
 
 ## Status (rolling)
 
-`bash lib/commerce/conformance.sh` → **228/228** (14 suites; + window) — **roadmap complete; Phase 5 extensions in progress**
+`bash lib/commerce/conformance.sh` → **239/239** (15 suites; + nettax) — **roadmap complete; Phase 5 extensions in progress**
 
 ## Ground rules
 
@@ -84,8 +84,9 @@ that unlocks the most tests per effort each iteration.
 - [x] time-windowed promotions — `window.sx`: windowed promo `(promo from until)`,
   `active-ruleset`/`active-codes`/`windowed-quote` gate by datetime; feeds the
   existing promo/stack/quote pipeline unchanged. Determinism preserved.
-- [ ] discount-aware tax policy — alternative `cart-quote` computing tax on the
-  net (post-discount) base via proportional class allocation; explicit + tested.
+- [x] discount-aware tax policy — `nettax.sx`: `cart-quote-net` taxes the net
+  (post-discount) base; `allocate-discount` spreads the basket discount across
+  lines by extended share with largest-remainder so per-line shares sum exactly.
 - [ ] refund as a flow — refund lifecycle (request → approve → settle) as a second
   flow-on-sx flow, recorded in the ledger; idempotent.
 - [ ] stock-constrained reservation — order-begin! fails (railway `fail`) when
@@ -97,6 +98,13 @@ that unlocks the most tests per effort each iteration.
   agnostic; `order-settle!(ref, amount)` is the resume seam.
 
 ## Progress log
+- 2026-06-07 — `nettax.sx` (Phase 5 ext): discount-aware tax — the alternative to
+  quote.sx's gross-tax default. `cart-quote-net` taxes the NET (post-discount)
+  base. `allocate-discount` spreads the basket-level discount across lines in
+  proportion to extended price with a deterministic largest-remainder pass so
+  per-line shares sum EXACTLY to the discount; each line is then taxed on its net
+  at its class rate. Both policies reproducible from inputs; pick per jurisdiction.
+  nettax suite 11/11; total 239/239 (15 suites).
 - 2026-06-07 — `window.sx` (Phase 5 ext): time-windowed promotions. A validity
   window is kept SEPARATE from the promo tuple — windowed promo `(promo from
   until)` (inclusive int timestamps, nil = open bound). `active-ruleset` filters