A cancellation handle for aborting an in-flight Butteraugli.compare/5 or Butteraugli.Reference.compare/3.

The ref is a neutral primitive — you assign it meaning. Create one with new/0, pass it as cancel: cancel_ref, and call Butteraugli.cancel/1 from any process (e.g. a client-disconnect monitor or a search deadline) to abort the comparison. The aborted call returns {:error, :cancelled}.

A ref is single-use: once cancelled it stays cancelled. One ref can cover a whole batch — cancelling it aborts the in-flight comparison and makes every subsequent comparison using it return {:error, :cancelled} at once.