W14: pin crit-2 signal-return kont non-vacuously (test-only)

crit-2's failure mode discards every frame outside the signal site —
including the covering test's own assert — which is why the shipped test
"signal returns handler value to call site" passed vacuously pre-fix. A
plain assert pin would inherit that vacuity on regression.

Add suite gate-crit2-signal-return-kont with a side-effect sentinel: test 1
runs the core.md repros ((list "outer" (handler-bind ... (+ 1
(signal-condition 5))) "end") -> ("outer" 43 "end"); raise-continuable ->
143) then set!s a top-level flag; test 2 independently asserts the flag, so
a dropped continuation fails loudly even though test 1 would "pass". Third
test pins the shipped-test expression (51). 267 passed / 0 failed under
OCaml run_tests.

Test-only: no semantics edits, no push.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

spec/tests/test-gate-pins.sx

@@ -161,3 +161,59 @@
         (quote (embed))
         (quote (param))
         (quote (track))))))
+
+;; --------------------------------------------------------------------------
+;; crit-2 [W1, critical] signal-return frame stored the saved kont under :f
+;; but the reader looked up "saved-kont" — the resume kont was always nil,
+;; so the handler value became the WHOLE program's result and every frame
+;; outside the signal site (including the covering test's own assert!) was
+;; silently discarded. The shipped test "signal returns handler value to
+;; call site" passed VACUOUSLY — the bug defeated its own test.
+;;
+;; A plain assert around the repro would inherit the same vacuity on
+;; regression (the dropped continuation includes the assert frame). So this
+;; pin uses a side-effect sentinel: test 1 runs the repro and then sets a
+;; flag; test 2 independently asserts the flag was reached. If crit-2
+;; regresses, test 1 still "passes" (vacuously) but test 2 FAILS.
+;; Repro (core.md).
+;; --------------------------------------------------------------------------
+(define *gate-crit2-after-signal* false)
+(define *gate-crit2-result* nil)
+(define *gate-crit2-rc-result* nil)
+
+(defsuite
+  "gate-crit2-signal-return-kont"
+  (deftest
+    "continuable signal resumes at the raise site"
+    (do
+      (set!
+        *gate-crit2-result*
+        (list
+          "outer"
+          (handler-bind
+            (((fn (c) true) (fn (c) 42)))
+            (+ 1 (signal-condition 5)))
+          "end"))
+      (set!
+        *gate-crit2-rc-result*
+        (handler-bind
+          (((fn (c) true) (fn (c) (+ c 100))))
+          (+ 1 (raise-continuable 42))))
+      (set! *gate-crit2-after-signal* true)
+      (assert= *gate-crit2-result* (list "outer" 43 "end"))
+      (assert= *gate-crit2-rc-result* 143)))
+  (deftest
+    "non-vacuity sentinel: the continuation after the signal actually ran"
+    (do
+      (assert
+        *gate-crit2-after-signal*
+        "continuation dropped — crit-2 regressed (previous test passed vacuously)")
+      (assert= *gate-crit2-result* (list "outer" 43 "end"))
+      (assert= *gate-crit2-rc-result* 143)))
+  (deftest
+    "handler value feeds the arithmetic frame, not the program result"
+    (assert=
+      (handler-bind
+        (((fn (c) true) (fn (c) (* c 10))))
+        (+ 1 (signal-condition 5)))
+      51)))