Christopher Lydon is the host of Open Source, a public-radio conversation with Internet roots and links. In the background now are too many years in straight journalism, covering city and state politics for the Boston Globe and presidential campaigns (McGovern, Humphrey, Reagan, Carter et al.) for the New York Times, anchoring The Ten O'Clock News at WGBH-TV in Boston, and founding The Connection with Mary McGrath at WBUR. Chris Lydon had run for mayor of Boston in 1993 -- a spur-of-the-moment stab at making a difference, or perhaps just surprising himself. In 1994, it turned out, the surprise that made the difference was radio.