An excavator ripped through the decorative cornices, arched windows, and bricks laid a century ago. The demolition of historic houses in a once-storied section of the city might bring despair for some, but Cornish, 74, said it is cause for celebration in her neighborhood. The long vacant houses being torn down are boarded up, missing roofs and windows. And they’re magnets for crime — from drug dealing to illegal dumping. On the site of the torn down houses, new townhomes are to be built, overlooking a new 3-acre park and playground.