BPC Throws a Party   Battery Park City’s block party, held Sept. 28, was an extraordinary event. It wasn’t the square dancing, pie eating contest, kiddie rides, arts and crafts or music that made it special. It was the coming together of a community in celebration of its own survival. Who could have imagined such a party in the dark days of the recent past, when every one of the neighborhood’s 9,000 residents were rendered homeless. “You could see Ground Zero down Vesey Street and no one was focusing on it. They just seemed to be having a great time,” said Angela Benfield, who helped organize the event. “It was a significant turning point in the atmosphere of the neighborhood and now we’re ready to move on.”