Fix IO suspension in both cek_run and cek_run_iterative

The _cek_io_suspend_hook was only added to cek_run_iterative (line 986)
but the actual code path went through cek_run (line 624). Added the hook
check to both functions.

This fixes the "IO suspension in non-IO context" error that blocked
hs-wait/perform from propagating through event handler → trampoline →
eval_expr call chains. IO suspension now converts to VmSuspended via the
hook, which the value_to_js wrapper catches and drives with _driveAsync.

+42 OCaml test passes (3924→3966). IO suspension verified working in
browser WASM: dom-on click handler → hs-wait → perform → suspend →
_driveAsync → setTimeout → resume.

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

hosts/ocaml/lib/sx_ref.ml

@@ -621,7 +621,9 @@ and cek_run state =
         let result = resolver request final in
         run (cek_resume final result)
       | None ->
-        raise (Eval_error (value_to_str (String "IO suspension in non-IO context")))
+        (match !Sx_types._cek_io_suspend_hook with
+         | Some hook -> hook final
+         | None -> raise (Eval_error (value_to_str (String "IO suspension in non-IO context"))))
     else cek_value final
   in run state