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_vm.ml

@@ -1020,6 +1020,17 @@ let () = _vm_suspension_to_dict := (fun exn ->
       | _ -> Nil));
     Some (Dict d)
   | _ -> None)
+(* Hook: when eval_expr (cek_run_iterative) encounters a CEK suspension,
+   convert it to VmSuspended so it propagates to the outer handler
+   (value_to_js wrapper, _driveAsync, etc.). Without this, perform
+   inside nested eval_expr calls (event handler → trampoline → eval_expr)
+   gets swallowed as "IO suspension in non-IO context". *)
+let () = _cek_io_suspend_hook := Some (fun suspended_state ->
+  let request = Sx_ref.cek_io_request suspended_state in
+  let vm = create (Hashtbl.create 0) in
+  vm.pending_cek <- Some suspended_state;
+  raise (VmSuspended (request, vm)))
+
 let () = _cek_eval_lambda_ref := (fun f args ->
   let state = Sx_ref.continue_with_call f (List args) (Env (make_env ())) (List args) (List []) in
   let final = Sx_ref.cek_step_loop state in