key 'nint' incorrectly places the limits when \limits is used

The `nint` key works by replicating `\int` as many times as needed, but the limits will only be *attached* to the last symbol. If `\limits` is applied it looks visually unappealing This can easily be fixed by using `\mathop`.

<img width="172" height="139" alt="Image" src="https://github.com/user-attachments/assets/d8ab76cc-70f2-49b0-ab63-dabd8f13e426" />