WESTCalifornia. Rosario Marin stepped down Monday as the 41st treasurer of the United States, paving the way for a possible challenge in 2004 to Democratic Sen. Barbara Boxer. Marin, a Republican, formerly served in GOP Gov. Pete Wilson's administration as legislative analyst for the Department of Developmental Services. She was also mayor of Huntington Park. California political consultant Kevin Spillane said Marin would make an announcement this month about running against Boxer, whose lukewarm fundraising and liberal voting record have made her a prime Republican target in 2004. Toni Casey, the Republican mayor of Los Altos Hills, already has announced...