The state Senate decided today to set an April 3 primary and a May 1 final election to select a replacement for Linda Dorcena Forry, who resigned on Friday as the senator for the 1st Suffolk District (South Boston and Dorchester). Whoever wins will then have to decide whether to run for re-election in the regular primary and general election this fall. Nomination papers are already available at the Secretary of State's offce at Ashburton Place.