All he had to do was show up. Donald Trump’s former adviser Peter Navarro looks set to become a prisoner next week after a final appeal against a contempt of Congress conviction failed Thursday — and a legal expert said it’s a sentence that would have been so simple for him to avoid. Navarro was found guilty of defying a subpoena demanding he supply documents and appear for deposition before the House committee investigating Jan. 6. He had been serving as White House trade adviser under Trump at the time, and peddled unfound conclusions of widespread voter fraud in the 2020 election defeat to …