Crown Heights in April: the Eastern Parkway malls go green almost overnight and every front yard wants attention at once. GREEN WALK PASTURE'S answers with mowing, edging, hedge trimming and the long spring clearing that carries off whatever winter left behind. The name is a small joke about the city, since pasture is exactly what a Brooklyn yard is not, and yet here it is, cut and raked and breathing. Fertilizing and seeding follow, so the thin patches fill in before July turns the heat up. A well-kept strip of green in front of a limestone row house is a kind of civic gift.