Canadian lumber producers' shares soar on strong demand, prices, fueled by better-than-expected US housing market indicators, surge in home improvement projects and tight lumber availability; Canfor shares up 60% from March low, West Fraser up 75%  Canadian lumber stocks have produced spectacular returns for investors during the stock market rebound since March. For evidence that the gains are sustainable, look south of the border.  Lumber prices are connected to U.S. home building activity, providing a link to the economy that is more direct, and immediate, than many other commodities.  Lumber ...