North American lumber headed for its biggest weekly loss since July as transportation snarls that sparked a rally start to ease.  Futures tumbled to $974.80 per 1,000 board feet on the Chicago Mercantile Exchange on Friday, heading for a sixth straight decline. That’s the longest slump since January. The pullback signals soaring costs are crimping consumer demand, just as more timber supplies are set to hit the market.  The price decline is tied to the DIY (do-it-yourself) sector slowing down due to high lumber prices, with people spending their money on ...