Peach, Prosciutto, and Goat-Cheese Pizzas

1 (1/4-oz) package active dry yeast

1 teaspoon sugar

1 cup warm water (110-115 F)

2 

1/2 cups all-purpose flour

1 teaspoon salt For topping

3 tablespoons extra-virgin olive oil

1 

1/2 firm-ripe peach es, cut into thin wedges and halved crosswise

8 very thin prosciutto slices, torn into pieces

3 oz soft mild goat cheese , crumbled

2 teaspoons chopped fresh rosemary

Special equipment: a pizza stone and baker's peel