Fix: replace guard with cek-try in test runner, clear stale reuse_stack

The guard form (call/cc + handler-bind expansion) doesn't survive async
IO suspension — the CEK continuation from guard's call/cc captures frames
that become invalid after the VM resumes from hs-wait. Replacing guard
with cek-try (which compiles to VM-native OP_PUSH_HANDLER/OP_POP_HANDLER)
avoids the CEK boundary crossing.

The test runner now executes: suspends on hs-wait, resumes, runs test
actions, and test assertions fire correctly. The "Not callable: nil"
error is eliminated. Remaining: test assertion errors from iframe content
not loading fast enough (timing issue, not a framework bug).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/lib/sx_vm.ml

@@ -854,11 +854,12 @@ let resume_vm vm result =
   (try run vm
    with VmSuspended _ as e ->
      (* Re-suspension during resume: the VM hit another perform.
-        Clear reuse_stack — these entries are stale from the PREVIOUS
-        suspension and don't apply to the current VM frame state.
-        The new VmSuspended carries the current VM state correctly. *)
-     vm.reuse_stack <- [];
+        The new VmSuspended carries the current VM state. *)
      raise e);
+  (* Clear reuse_stack — any entries here are stale from the original
+     suspension and don't apply to the current state. The VM just
+     completed its execution successfully. *)
+  vm.reuse_stack <- [];
   (* Restore call_closure_reuse continuations saved during suspension.
      reuse_stack is in catch order (outermost first from prepend) —
      reverse to get innermost first, matching callback→caller unwinding. *)

shared/static/wasm/sx_browser.bc.wasm.js

@@ -1792,7 +1792,7 @@
             blake2_js_for_wasm_create: blake2_js_for_wasm_create};
    }
    (globalThis))
-({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["re-9a0de245",[2]],["sx-7f504b3c",[2,3]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,5]],["dune__exe__Sx_browser-b285d4f3",[2,4,6]],["std_exit-10fb8830",[2]],["start-f808dbe1",0]],"generated":(b=>{var
+({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["re-9a0de245",[2]],["sx-267801d6",[2,3]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,5]],["dune__exe__Sx_browser-b285d4f3",[2,4,6]],["std_exit-10fb8830",[2]],["start-f808dbe1",0]],"generated":(b=>{var
 c=b,a=b?.module?.export||b;return{"env":{"caml_ba_kind_of_typed_array":()=>{throw new
 Error("caml_ba_kind_of_typed_array not implemented")},"caml_exn_with_js_backtrace":()=>{throw new
 Error("caml_exn_with_js_backtrace not implemented")},"caml_int64_create_lo_mi_hi":()=>{throw new

sx/sx/applications/htmx/runner.sx

@@ -105,13 +105,11 @@
                   (reset! current (str "Running: " name))
                   (reset! results (assoc (deref results) name "running"))
                   (reload-frame)
-                  (guard
-                    (e
-                      (true
-                        (reset! results (assoc (deref results) name "fail"))
-                        (console-log (str "FAIL " name ": " e))))
-                    (for-each run-action (get test :actions))
-                    (reset! results (assoc (deref results) name "pass")))))
+                  (let
+                    ((test-ok (cek-try (fn () (for-each run-action (get test :actions)) true) (fn (e) (do (reset! results (assoc (deref results) name "fail")) (console-log (str "FAIL " name ": " e)) false)))))
+                    (when
+                      test-ok
+                      (reset! results (assoc (deref results) name "pass"))))))
               tests)
             (reset! running false)
             (reset! current "Done"))))