Cut the rhubarb into 1-inch pieces and put in a small saucepan with 1/4 cup water. Cover and bring to a boil, then decrease the heat and simmer, stirring occasionally. When the rhubarb starts to soften, after about 5 minutes, stir in the cardamom, nutmeg, and salt. Break up any large pieces of rhubarb with a wooden spoon. Continue to simmer, covered, until the rhubarb is completely softened, about 4 minutes more. Remove from the heat and stir in the honey and vanilla extract. Let cool. Add the rose water. To serve, put 1/2 cup of yogurt in each bowl and top with a few tablespoons of the rhubarb. Scatter a few tablespoons of pistachios over the top.