function getDeltaLabel(delta, deltaType = 'audit') {
  if (delta === 0) return 'neutral';
  let isImprovement = delta < 0;
  if (deltaType === 'score') isImprovement = delta > 0;
  return isImprovement ? 'improvement' : 'regression';
}
@param {number} delta
@param {'audit'|'score'} deltaType
@return {DiffLabel}