3 stars; Rated PG-13; 182 minutes If the staggering losses in “Avengers: Infinity War” gave the Marvel Cinematic Universe an inevitable and operatic sense of climax, “Avengers: Endgame” shows what it’s like after the curtain has fallen. The sequel to last year’s “Infinity War” returns to just before Thanos snapped his fingers, bringing about a shocking conclusion to the demigod’s quest for the Infinity Stones.