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>

plans/agent-briefings/sx-gate-loop.md

@@ -50,7 +50,8 @@ Pin each confirmed-and-fixed finding with a minimal repro. Add suites to
 - [x] K49 [W8] — five void elements (area/base/embed/param/track) renderable
       (spec side; native regen drift → see Blocked). NB: the depth/cycle guard
       is K16 [W8], still OPEN — not a W14 pin target until its fix lands
-- [ ] crit-2 [W1] — signal-return frame key (verify the pin is non-vacuous)
+- [x] crit-2 [W1] — signal-return kont pinned NON-VACUOUSLY (side-effect
+      sentinel across two tests; a plain assert would inherit the vacuity)
 - [ ] C1/C1b [W3] — HTTP-mode concurrency fixes, pin
 - [ ] S4 [conformance] — housekeeping repro, pin
 
@@ -79,6 +80,16 @@ Pin each confirmed-and-fixed finding with a minimal repro. Add suites to
 
 ## Progress log (newest first)
 
+- 2026-07-04 — **crit-2 non-vacuous pin (item A.5)**. The original bug's
+  signature — handler value becomes the WHOLE program result, discarding
+  every outer frame *including the covering test's own assert* — means a
+  plain `(assert= repro expected)` pin would pass vacuously on regression.
+  Added suite `gate-crit2-signal-return-kont` with a **side-effect sentinel**:
+  test 1 runs both repros (`("outer" 43 "end")` list shape + `raise-continuable`
+  → 143) then `set!`s a top-level flag; test 2 independently asserts the flag
+  — if the continuation is ever dropped again, test 1 "passes" but test 2
+  fails loudly. Third test pins the exact shipped-test expr (51). Verified
+  both repro shapes live via sx_eval first. 267 passed / 0 failed. Test-only.
 - 2026-07-03 — **K49 void-elements pin (item A.4) + regen-drift DISCOVERY**.
   Corrected the checklist label first: K49 is "five void elements
   unrenderable" (core.md:335), not the depth guard (that's K16, OPEN). Added