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.json

@@ -14,9 +14,10 @@
     "attribution": {"pass": 16, "fail": 0},
     "payment": {"pass": 7, "fail": 0},
     "window": {"pass": 19, "fail": 0},
-    "nettax": {"pass": 11, "fail": 0}
+    "nettax": {"pass": 11, "fail": 0},
+    "stock": {"pass": 19, "fail": 0}
   },
-  "total_pass": 239,
+  "total_pass": 258,
   "total_fail": 0,
-  "total": 239
+  "total": 258
 }