Canada's worst-ever spring wildfire season has forced its forestry industry to shutter sawmills, driving up lumber prices and setting production back for months just as housing construction has slowed due to higher costs and a tight labor market.    This year's unprecedented fires have already consumed at least 4 million hectares, or 1% of Canada's forest, according to the Forest Products Association of Canada (FPAC), an industry group.    The fires are blazing through Alberta, British Columbia and Quebec, all provinces with active forestry industries.    The ...