```ts
/** Truncates the text to limit characters with an ellipsis when longer; throws if limit < 3. */
export function truncate(text: string, limit: number): string {
  if (typeof limit !== 'number' || limit < 3) {
    throw new Error("Limit must be a positive integer of at least 3.");
  }
  if (text.length <= limit) {
    return text;
  }
  return text.slice(0, limit - 3) + "...";
}
```