Plunging North American lumber prices may have sent shares of producers tumbling in recent weeks, but analysts and traders say that if history is any guide, stocks are set for a rebound as the homebuilding season picks up into year-end.  Over the past 20 years, lumber prices and shares of companies from West Fraser Timber Co. and  Canfor Corp. to Louisiana-Pacific Corp. have tended to slump in September before rallying into the new year, according to a Bloomberg analysis of seasonality patterns. RBC Capital Markets analyst Paul Quinn says don’t expect this go-around to ...