Bruce F. Romer has been beyond and returned to tell about it. On Dec. 21, 1998, Montgomery County, Md., conducted a full-scale year-2000 exercise. Computer-aided 911 dispatch, traffic light control systems, accounts payable and payroll, and voter registration systems were tested in realtime with clocks rolled ahead. Many lessons were learned, said Romer, the county's chief administrative officer. He knows a little about Y2K, having testified before Congress and briefed the White House Y2K staff. But Romer's range extends beyond the county