Sergei Loznitsa’s “Two Prosecutors” is a nightmare of government corruption so perfectly composed that, by the time it reaches its chilling conclusion, you feel nearly as entrapped as its young protagonist. Alexander Kornyev (Aleksandr Kuznetsov) is a fresh-faced prosecutor who arrives at the Soviet prison in Bryansk in 1937. This is not, you might be thinking, where anyone new to the job should be in a rush to get to. Yet before even the news of his appointment has reached this penal outpost,