US lumber futures have fallen to their weakest level in around two years, trading close to $530 per 1,000 board feet as high borrowing costs continue to weigh on housing demand.    The decline extends a slide that began in late July. Futures had been near $535 on September 22, roughly 20% below their July peak, and the market has continued to soften since then.    The main pressure is still coming from housing. Mortgage rates have climbed to around 7.3%, their highest level in three years, while higher bond yields and ...