Bring a large pot of salted water to a boil. Add the spaghetti and cook as the label directs. Reserve about 1/2 cup cooking water, then drain in a colander and rinse under cold water. Shake off the excess water. Meanwhile, puree the peanut butter, curry paste, vinegar, lime juice, cilantro, 1/2 cup water and 1 teaspoon salt in a food processor or blender until smooth. Toss the spaghetti with the peanut sauce, scallions, cucumber and carrot in a large bowl until coated. Season with salt and stir in some of the reserved cooking water to loosen the sauce, if necessary. Transfer to bowls and top with more cilantro and pepper flakes, if desired. Serve with lime wedges.