IO suspension: _cek_io_suspend_hook propagates perform through eval_expr

Root cause: cek_run_iterative (used by eval_expr/trampoline) raised
"IO suspension in non-IO context" when the CEK hit a perform. This
blocked IO suspension from propagating through nested eval_expr calls
(event handler → trampoline → eval_expr → for-each callback → hs-wait).

Fix: added _cek_io_suspend_hook (Sx_types) that converts CEK suspension
to VmSuspended, set by sx_vm.ml at init. cek_run_iterative now calls the
hook instead of erroring. The VmSuspended propagates to the value_to_js
wrapper which has _driveAsync handling.

+42 test passes (3924→3966), zero regressions.

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

hosts/ocaml/lib/sx_ref.ml

@@ -983,7 +983,15 @@ let cek_run_iterative state =
       s := cek_step !s
     done;
     (match cek_suspended_p !s with
-     | Bool true -> raise (Eval_error "IO suspension in non-IO context")
+     | Bool true ->
+       (* Propagate IO suspension so the outer handler
+          (value_to_js wrapper or _cek_eval_lambda_ref) can drive it.
+          Without this, perform inside nested eval_expr contexts
+          (e.g. event handler → trampoline → eval_expr) gets swallowed.
+          Use _cek_io_suspend_hook if set; otherwise fall back to error. *)
+       (match !Sx_types._cek_io_suspend_hook with
+        | Some hook -> hook !s
+        | None -> raise (Eval_error "IO suspension in non-IO context"))
      | _ -> cek_value !s)
   with Eval_error msg ->
     _last_error_kont_ref := cek_kont !s;