Bond yields sank and stocks recovered a fraction of their weekly losses on Friday, kicking off the back half of a year that more investors fear could be marred by a slowing economy. A day after closing out the index’s worst first half of any year since 1970, the S&P 500 on Friday clawed its way into the green after early losses but still finished the week down 2.2%. It has now fallen for 11 of the past 13 weeks.