The New York State appellate court decision yesterday to allow the controversial renovation of Washington Square Park to go forward will likely have an unintended side-effect that will make the recent Kentucky Fried Rat infestation just around the corner look like, well, a day in the park. Ever since 9-11, when rats surged away from ground zero through subway tunnels, the area around Washington Square has been infested with them. They party on street corners, feast on garbage and cavort (and breed) in neighboring gardens like so many randy club kids. And recent building in the area, including the excavations of several basements on Waverly Place and the new condo that just rose on the corner of Washington Place and Sixth Avenue destroyed decades’ old rat tunnels and sent more packs of the nasty critters racing around the neighborhood. More still —many more —make the famous fenced-off skateboard mounds in Washington Square their private Rodent Motel. When the demolition of those mounds begins, the present will look like a most pleasant prologue. Stop, hey, what’s that sound? The squeals of newly homeless rats? Or those of the newbie homeowners paying millions to live downtown? Come to think of it, some of them may well feel more at home among the rodents.