Carnival. . . Mardi Gras . . . Fat Tuesday. . . the gateway to the Lenten season, when worldly pleasures are forsworn. For centuries it's been a time of celebration when the world turns upside down and the rules run riot in an excess of food and music, costume and revelry. In this program,Sound & Spirit host Ellen Kushner follows the festival around the world and through the ages, from its medieval origins to the 19th-century German carnival music of Robert Schumann; to the samba schools of Brazil and Haiti's frenetic celebrations; to the African American Mardi Gras "Indians" of New Orleans.