Another week of trade threats and promised retaliation kept investors off stride and markets adrift. The S&P 500 shed 0.9 percent, its first weekly decline in the past five. Trade jitters were even less kind to the Dow Jones Industrial Average, however. Owing to its constituents’ greater exposure to foreign trade, the Dow fell 2.0 percent last week, its third decline in the past four weeks. And only a modest bounce on Friday prevented the Dow from dropping for nine straight days.