WASHINGTON — The presidents of America and Iran may meet briefly next week for the first time, marking a symbolic but significant step toward easing their countries' tense relationship. An exchange of letters between the leaders already has raised expectations for a thaw in relations, and any progress in dismantling Syria's chemical weapons stockpile could signal whether their elusive diplomacy will last longer than a handshake. At the heart of the U.S.-Iran impasse is a years-long dispute over Tehran's nuclear program.