commerce: stock-constrained reservation (19 tests) — Phase 5 ext

stock.sx — reservation as a precondition the host checks before order-begin!
(validate -> begin), keeping the flow pure. available-stock reads catalog stock
facts; can-reserve?/reserve-check/reservation-shortfalls gate a cart;
effective-available nets out concurrent reservations so orders can't
over-reserve; sufficient-stocko is the multidirectional availability query.
Total 258/258 across 16 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 nettax)
+SUITES=(catalog cart price api promo stack quote ledger order recon federation attribution payment window nettax stock)
 
 OUT_JSON="lib/commerce/scoreboard.json"
 OUT_MD="lib/commerce/scoreboard.md"
@@ -68,6 +68,7 @@ run_suite() {
 (load "lib/commerce/quote.sx")
 (load "lib/commerce/window.sx")
 (load "lib/commerce/nettax.sx")
+(load "lib/commerce/stock.sx")
 (load "lib/commerce/ledger.sx")
 (load "lib/commerce/order.sx")
 (load "lib/commerce/payment.sx")