May 4 may well be remembered as the happiest day in D.C. Mayor Anthony Williams’ political career. It is today, at 10 a.m., that Williams and new Nationals owner Ted Lerner will come together to break ground on the new $611 million stadium in Southeast. The Nats may have come back to Washington in September 2004, but their longevity in the city was consistently tested by bruising battles over stadium financing and team ownership,…