Fix VM reuse_stack lost across stub VM boundary on IO suspension

Root cause: when perform fires inside a VM closure chain (call_closure_reuse),
the caller frames are saved to reuse_stack on the ACTIVE VM. But the
_cek_io_suspend_hook and _cek_eval_lambda_ref create a NEW stub VM for the
VmSuspended exception. On resume, resume_vm runs on the STUB VM which has
an empty reuse_stack — the caller frames are orphaned on the original VM.

Fix: transfer reuse_stack from _active_vm to the stub VM before raising
VmSuspended. This ensures resume_vm -> restore_reuse can find and restore
the caller's frames after async resume via _driveAsync/setTimeout.

Also restore step_limit/step_count refs dropped by bootstrap.py regeneration.

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

hosts/ocaml/lib/sx_vm.ml

@@ -1029,6 +1029,13 @@ let () = _cek_io_suspend_hook := Some (fun suspended_state ->
   let request = Sx_ref.cek_io_request suspended_state in
   let vm = create !_default_vm_globals in
   vm.pending_cek <- Some suspended_state;
+  (* Transfer reuse_stack from the active VM so resume_vm can restore
+     caller frames saved by call_closure_reuse during the suspension chain. *)
+  (match !_active_vm with
+   | Some active when active.reuse_stack <> [] ->
+     vm.reuse_stack <- active.reuse_stack;
+     active.reuse_stack <- []
+   | _ -> ());
   raise (VmSuspended (request, vm)))
 
 let () = _cek_eval_lambda_ref := (fun f args ->
@@ -1040,6 +1047,12 @@ let () = _cek_eval_lambda_ref := (fun f args ->
        resume_vm will: cek_resume → push result → run (no-op, no frames) → pop *)
     let vm = create (Hashtbl.create 0) in
     vm.pending_cek <- Some final;
+    (* Transfer reuse_stack from active VM *)
+    (match !_active_vm with
+     | Some active when active.reuse_stack <> [] ->
+       vm.reuse_stack <- active.reuse_stack;
+       active.reuse_stack <- []
+     | _ -> ());
     raise (VmSuspended (Sx_runtime.get_val final (String "request"), vm))
   | _ -> Sx_ref.cek_value final)