Empty shelves, grumpy customers, long lines at the register: These are just some of the challenges facing Wal-Mart Stores Inc. (WMT)’s new U.S. chief. Greg Foran, who takes charge Aug. 9, inherits a chain wedded to an outdated big-box model, struggling to increase sales in its stores and online, and losing customers because it can’t keep stores adequately stocked. The sluggish U.S. performance contributed to lower-than-projected sales and profit in the quarter that ended April 30. Wal-Mart’s profit forecast for the current quarter also came in lower than analysts’ estimates.