Rhubarb Lambrusco Granita

Bring wine to a boil with water and sugar, stirring, in a 4-quart heavy pot. Add rhubarb and cook at a bare simmer, covered, stirring occasionally, until very tender and beginning to fall apart, 5 to 7 minutes. Purée in a food processor with lemon juice until smooth, then force through a coarse sieve into an 8- to 9-inch baking pan, pressing hard on solids (discard solids). Freeze, stirring and crushing lumps with a fork every 1 1/2 hours, until evenly frozen, about 8 hours total. Scrape granita with a fork to lighten texture, crushing any lumps. Serve immediately or freeze, covered, up to 3 days (rescrape to lighten texture again if necessary).