"Third, ...we determined not to apply an approach that could potentially result in a judgment that the President committed crimes. The threshold step under the Justice Manual standards is to assess whether a person's conduct " constitutes a federal offense." ...Fairness concerns counseled against potentially reaching that judgment when no charges can be brought. The ordinary means for an individual to respond to an accusation is through a speedy and public trial, with all the procedural protections that surround a criminal case. ...In contrast, a prosecutor's judgment that crimes were committed, but that no charges will be brought, affords no such adversarial opportunity for public name-clearing before an impartial adjudicator."