WASHINGTON — The legal sparring around Donald Trump’s impeachment trial is underway, with briefs filed this week laying out radically different positions ahead of next week’s Senate trial.House prosecutors and the former president’s defense team are putting forward their arguments about Trump’s role in the Jan. 6 riot at the U.S. Capitol and on the legality of even holding a trial. They’re also debating the First Amendment and a blunt assessment by Democrats that the riot posed a threat to the presidential line of …