1. In a 3- to 4-quart pan, combine broth, catsup, ginger, mustard seeds, chili powder, garam masala, and dill weed. Bring to a boil over high heat; when boiling, add shrimp and cook until opaque but still moist-looking in center of thickest part (cut to test), 3 to 4 minutes.
2. At once, stir couscous into pan, cover, and remove from heat. Let stand 5 minutes. Stir mixture with a fork; there should be some liquid. Spoon shrimp couscous onto plates; sprinkle with mint and add yogurt, salt, and juice from lime wedges.