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;

hosts/ocaml/lib/sx_types.ml

@@ -252,6 +252,12 @@ exception CekPerformRequest of value
     Used by the HTTP server to handle perform (text-measure) during aser. *)
 let _cek_io_resolver : (value -> value -> value) option ref = ref None
 
+(** Hook: handle CEK IO suspension in eval_expr (cek_run_iterative).
+    When set, called with the suspended CEK state instead of raising
+    "IO suspension in non-IO context". Used by the browser WASM kernel
+    to convert CEK suspensions to VmSuspended for _driveAsync handling. *)
+let _cek_io_suspend_hook : (value -> value) option ref = ref None
+
 (** Hook: convert VM suspension exceptions to CekPerformRequest.
     Set by sx_vm after it defines VmSuspended. Called by sx_runtime.sx_apply_cek. *)
 let _convert_vm_suspension : (exn -> unit) ref = ref (fun _ -> ())

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