Shortly after he declared that he would have run into a Florida high school unarmed to thwart a mass shooting, Donald J. Trump announced that he was planning a parade in Washington, D.C., to celebrate his hypothetical act of heroism. “Anyone can act with bravery in the moment,” Trump told reporters in the White House. “But it takes a very special kind of hero to tell people about the incredibly brave thing he would have done weeks after the thing happened.”