SHAMUKJULI, India — Hundreds of survivors of a brutal rebel attack that killed at least 63 people in northeastern India sought shelter Wednesday in a church and school while security forces imposed a curfew in a bid to contain the latest bout of ethnic violence. Long-simmering land and ethnic disputes in Assam state erupted in bloodshed Tuesday when authorities said rebels belonging to a faction of an indigenous separatist group called the National Democratic Front of Bodoland…