Lambda→CEK dispatch: enable IO suspension through sx_call

Lambda calls in sx_call now go through the CEK machine instead of
returning a Thunk for the tree-walker trampoline. This lets perform/
IO suspension work everywhere — including hyperscript wait/bounce.

Key changes:
- sx_runtime: Lambda case calls _cek_eval_lambda_ref (forward ref)
- sx_vm: initializes ref with cek_step_loop + stub VM for suspension
- sx_apply_cek: VmSuspended → __vm_suspended marker dict (not exception)
- continue_with_call callable path: handles __vm_suspended with
  vm-resume-frame, matching the existing JIT Lambda pattern
- sx_render: let VmSuspended propagate through try_catch
- Remove invalid io-contract test (perform now suspends, not errors)

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

hosts/ocaml/lib/sx_runtime.ml

@@ -44,10 +44,8 @@ let sx_call f args =
   match f with
   | NativeFn (_, fn) -> fn args
   | VmClosure cl -> !Sx_types._vm_call_closure_ref cl args
-  | Lambda l ->
-    let local = Sx_types.env_extend l.l_closure in
-    List.iter2 (fun p a -> ignore (Sx_types.env_bind local p a)) l.l_params args;
-    Thunk (l.l_body, local)
+  | Lambda _ ->
+    !Sx_types._cek_eval_lambda_ref f args
   | Continuation (k, _) ->
     k (match args with x :: _ -> x | [] -> Nil)
   | CallccContinuation _ ->
@@ -75,11 +73,22 @@ let sx_apply_cek f args_list =
   match f with
   | NativeFn _ | VmClosure _ ->
     (try sx_apply f args_list
-     with Eval_error msg ->
-       let d = Hashtbl.create 3 in
-       Hashtbl.replace d "__eval_error__" (Bool true);
-       Hashtbl.replace d "message" (String msg);
-       Dict d)
+     with
+     | CekPerformRequest _ as e -> raise e
+     | exn ->
+       (* Check if this is a VM suspension — return marker dict so
+          continue_with_call can build a proper suspended CEK state
+          with vm-resume-frame on the kont. *)
+       (match !_vm_suspension_to_dict exn with
+        | Some marker -> marker
+        | None ->
+          (match exn with
+           | Eval_error msg ->
+             let d = Hashtbl.create 3 in
+             Hashtbl.replace d "__eval_error__" (Bool true);
+             Hashtbl.replace d "message" (String msg);
+             Dict d
+           | _ -> raise exn)))
   | _ -> sx_apply f args_list
 
 (** Check if a value is an eval-error marker from sx_apply_cek. *)