Toss the carrots and cauliflower with 3 tablespoons of the oil on a rimmed baking sheet. Combine the curry powder, 1/4 teaspoon each of the coriander and cumin, 3/4 teaspoon salt, and a few grinds of pepper and sprinkle over the vegetables. Spread in a single layer and roast, stirring once or twice, until crisp-tender and lightly charred in spots, 20 to 25 minutes. Transfer to a large bowl and let cool slightly.
Bring 2/3 cup water to a boil. Add the couscous, the remaining 1/4 teaspoon each coriander and cumin, and a pinch of salt. Remove from the heat, cover, and let stand for 10 minutes.
Stir together the yogurt, 1/4 cup water, lime juice, remaining 1 tablespoon oil, and 1/2 teaspoon salt.
Mix the roasted vegetables with the couscous. Drizzle in half the dressing and toss to coat. Toss the greens in the remaining dressing with salt to taste and put on top of the couscous mixture.
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