Behind the charming boutiques of Main Street in Manayunk sits a crude shell of a building. The property, which runs along the Manayunk Canal on a strip of land called Venice Island, is what’s left of a 19th-century textile mill. It’s basically four jagged, graffiti-covered walls with no roof and nothing inside — as if someone had started demolishing from the top and worked down, but never finished. “It’s pretty gruesome,” says Mike Yanofsky, who works on Main Street and was taking an afternoon stroll along the canal’s boardwalk when we caught up with him.