U.S. lawmakers, after shrugging off a White House veto threat, were en route today to providing $4.1 billion in emergency aid to farmers caught in an abrupt economic downturn. Farm income was forecast to plunge 13% this year, dragged down by an oversupply of meat and a global grain glut, along with losses from drought, hurricanes and wheat scab. Negotiators from the House and Senate Monday wrote the aid, split between disaster relief and bonus payments to farmers to make up for weak agriculture exports, into a bill providing $61.3 billion for the Agriculture Department and related agencies in fiscal 1999.