Professional flea market weasel John Maloof hit the jackpot when a purchase of some negatives at an estate sale lead him to discover Vivian Maier — just an ordinary French immigrant nanny who, in her spare time, was a prolific and gifted street photographer of ’50s and ’60s Chicago. Most of the collection (now Maloof’s) — 2,000 rolls of film, 3,000 prints, 100,000 negatives — has never been seen. Maier’s postmortem retrospective opens Friday at the Chicago Cultural Center.