Preheat oven to 350°F. and lightly butter a 9-inch springform pan. In a food processor finely grind graham crackers and amaretti with sugar. Add butter and blend until combined well. Press mixture onto bottom and about 3/4 inch up side of pan. Bake crust in middle of oven 10 to 15 minutes, or until crisp and golden, and cool completely in pan on a rack. In a bowl with an electric mixer beat mascarpone with sugar until smooth and add eggs, 1 at a time, beating well after each addition. Beat in lemon juice, zest, and extracts. Beat in flour and salt until combined well. Pour filling into crust. Bake tart in middle of oven 30 to 35 minutes, or until pale golden and set, and cool completely in pan on rack. Remove side of pan and transfer tart to a plate. Chill tart, covered, at least 4 hours and up to 24. Slice nectarines and in a bowl toss with sugar. Let nectarines stand, tossing occasionally, until sugar is dissolved and juices are released, about 5 minutes. Mound topping on tart and serve immediately.