As a homeowner uses her cell phone to scan the QR code on a direct-mail piece, a map that shows her precisely how to go from her home to the business that mailed the advertisement pops up on her cell phone.Or a personalized greeting "Hi, Ann!" shows up from the company's chief executive. Or maybe an offer to enter a contest in exchange for an e-mail address.Sophisticated marketers in northern Nevada are using personalized QR codes in greater numbers this summer, spurred in part by an effort by the U.S. Postal Service to show the capabilities of the new technology.The …