<p>When Mayor Willie Brown becomes "Citizen Brown'' at noon Thursday for the first time in four decades, the shift into his new life, he said, "is going to be traumatic -- but not disabling.''</p>
<p>After a groundbreaking 31-year career in the state Assembly followed by two terms as San Francisco's 41st mayor, Brown already is hatching his next moves: TV commentator, consultant, lawyer and head of a new public policy institute he's creating to train the next crop of politicians.</p>