The great Chinese stock slump that first whacked luxury car sales is spreading to mass-market brands as wannabe customers like Zhang Jiabin count the cost of soured investments. The 37-year-old food company executive lost nearly $6,500 when shares tumbled in June and July, and can’t now afford the new Volkswagen Tiguan sport-utility vehicle he had his eye on. Auto sales in China fell 7.1 percent in July from a year earlier as many more who lost out in a trillion-dollar share slump joined Zhang in delaying purchases.