BOSTON — Roberto Luongo came off the bench to shut down Boston in the third period, and the Florida Panthers dashed the Bruins’ hopes of getting the top seed in the Eastern Conference playoffs with a 4-2 win Sunday night. The Bruins trailed Tampa Bay by a point heading into the regular-season finale but couldn’t top a Panthers team that was eliminated from postseason contention Saturday. Boston will host the Toronto Maple Leafs in the first round beginning Thursday night.