Pat lamb dry. Heat oil in a 7- to 8-quart heavy pot over moderately high heat until hot but not smoking, then brown lamb in 3 batches, transferring to a bowl.
Add onions to pot and cook, stirring occasionally, until golden, about 10 minutes. Stir in lamb with any juices that have accumulated in bowl, tomatoes, tomato juice, salt, black pepper, chile, herbs, and saffron and simmer, covered, 1 1/2 hours.
Stir in carrots, turnips, bell peppers, and pumpkin and simmer, covered, 30 minutes. Stir in zucchini and chickpeas and simmer, covered, until zucchini is tender, 20 to 25 minutes.
Pour 1 cup broth from pot into a small heavy saucepan and add tomato paste, whisking until smooth. Simmer, stirring, until thick, about 10 minutes, then stir in paprika, rose petals, and cayenne.
Bring water to a boil with butter and salt in a 4-quart heavy saucepan. Stir in couscous, cover, and remove from heat. Let stand 5 minutes, then fluff with a fork. Mound couscous on a platter and top with stew, using a slotted spoon. Serve broth and tomato sauce on the side.
