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/tests/stock.sx

@@ -0,0 +1,122 @@
+;; lib/commerce/tests/stock.sx — stock-constrained reservation.
+;; Uses (commerce-test name got expected) provided by conformance.sh.
+
+(define
+  cat
+  (make-catalog
+    (list
+      (list "widget" 1000 :standard)
+      (list "gadget" 2500 :standard))
+    (list)
+    (list
+      (list "widget" :small 5)
+      (list "widget" :large 0)
+      (list "gadget" :std 12))))
+
+;; --- availability ---
+
+(commerce-test
+  "available-found"
+  (available-stock cat "widget" :small)
+  5)
+(commerce-test
+  "available-zero"
+  (available-stock cat "widget" :large)
+  0)
+(commerce-test
+  "available-absent"
+  (available-stock cat "widget" :none)
+  0)
+
+;; --- per-line reservability ---
+
+(commerce-test
+  "shortfall-fits"
+  (line-shortfall cat (list "widget" :small 5))
+  0)
+(commerce-test
+  "shortfall-over"
+  (line-shortfall cat (list "widget" :small 8))
+  3)
+(commerce-test
+  "reservable-yes"
+  (line-reservable? cat (list "gadget" :std 12))
+  true)
+(commerce-test
+  "reservable-no"
+  (line-reservable? cat (list "widget" :large 1))
+  false)
+
+;; --- cart-level reservation check ---
+
+(commerce-test
+  "can-reserve-yes"
+  (can-reserve?
+    cat
+    (list (list "widget" :small 5) (list "gadget" :std 2)))
+  true)
+
+(commerce-test
+  "can-reserve-no"
+  (can-reserve? cat (list (list "widget" :small 9)))
+  false)
+
+(commerce-test
+  "shortfalls-detail"
+  (reservation-shortfalls
+    cat
+    (list (list "widget" :small 9) (list "gadget" :std 2)))
+  (list {:requested 9 :available 5 :sku "widget" :variant :small :short 4}))
+
+(commerce-test
+  "reserve-check-ok"
+  (reserve-check cat (list (list "gadget" :std 1)))
+  :ok)
+
+(commerce-test
+  "reserve-check-rejected"
+  (reserve-check cat (list (list "widget" :large 1)))
+  {:shortfalls (list {:requested 1 :available 0 :sku "widget" :variant :large :short 1}) :rejected :insufficient-stock})
+
+;; --- reservation view: concurrent holds reduce availability ---
+
+(define held (list (list "widget" :small 3)))
+
+(commerce-test
+  "effective-after-hold"
+  (effective-available cat held "widget" :small)
+  2)
+(commerce-test
+  "effective-other-unaffected"
+  (effective-available cat held "gadget" :std)
+  12)
+(commerce-test
+  "reservable-with-fits"
+  (line-reservable-with? cat held (list "widget" :small 2))
+  true)
+(commerce-test
+  "reservable-with-over"
+  (line-reservable-with? cat held (list "widget" :small 3))
+  false)
+
+;; --- relational availability query (multidirectional) ---
+
+(commerce-test
+  "sufficient-forward"
+  (run*
+    x
+    (fresh () (sufficient-stocko cat "widget" :small 5) (== x true)))
+  (list true))
+
+(commerce-test
+  "sufficient-forward-over"
+  (run*
+    x
+    (fresh () (sufficient-stocko cat "widget" :small 6) (== x true)))
+  (list))
+
+;; backward: which variants of widget can supply 1 unit?
+(commerce-test
+  "variants-supplying-1"
+  (run* v (fresh (q) (stocko cat "widget" v q) (lteo-i 1 q)))
+  (list :small))