Investors recovered their poise after a shaky start sent stocks sharply lower, following a global slump. By midday stocks had recouped most of their earlier losses, and the day ended marginally lower. This year has been about steady gains, punctuated by infrequent declines, thanks to a pickup in hiring, a housing recovery that seems to be gaining steam, and record corporate profits. On Thursday, though, the trading was volatile. A global sell-off came after Fed meeting minutes showed some policy makers lean toward slowing the stimulus as early as June. And a report said that manufacturing in China shrank unexpectedly this month.