In a saucepan stir together the brown sugar, the cornstarch, the zest, and the juice and add the cranberries, the raisins and the salt. Bring the mixture to a boil, and simmer it, stirring for 5 minutes, or until the berries have just burst. Transfer the filling to a bowl and chill it covered, for 2 hours, or until it is cold. The filling may be made 2 days in advance and kept covered and chilled.