PRESIDENT BUSH confronts several knotty issues during his current Asian trip, among them free trade, nuclear proliferation, and the specter of avian influenza. But none may be more fateful in coming decades than the longterm US relationship with China. So it was encouraging that Bush’s remarks about China in a speech he delivered Wednesday in Kyoto balanced criticism of Beijing’s undemocratic practices with signals that the United States does not want to unravel the world’s most fraught bilateral relationship.