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_render.ml

@@ -64,6 +64,7 @@ let expand_macro m args_val _env = match m with
 let try_catch try_fn catch_fn =
   try sx_call try_fn []
   with
+  | Sx_vm.VmSuspended _ as e -> raise e
   | Eval_error msg -> sx_call catch_fn [String msg]
   | e -> sx_call catch_fn [String (Printexc.to_string e)]