The endgame in the trade war between China and the United States seems near. President Trump, betting with real currency — American strength — apparently has the upper hand, and the concessions President Xi Jinping is likely to make won’t be mere tokens. When — if? — an agreement is finally announced, Mr. Trump will surely fire off bragging tweets, partly to shore up his credentials for a second term, amid personal and policy troubles. For Mr. Xi, almost any deal could mean a very serious loss of face.