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/scoreboard.md

@@ -19,4 +19,5 @@ _Generated by `lib/commerce/conformance.sh`_
 | payment | 7 | 0 | 7 |
 | window | 19 | 0 | 19 |
 | nettax | 11 | 0 | 11 |
-| **Total** | **239** | **0** | **239** |
+| stock | 19 | 0 | 19 |
+| **Total** | **258** | **0** | **258** |