Seven years ago, comedian Jay Leno invited us into his mega-garage/museum in Burbank, not too far from the NBC studio where he once taped “The Tonight Show.” Everyone knows he’s very serious about his cars but, at the same time, he’s also just a kid enjoying his Matchbox miniatures every day, now in retirement: steamers, tire-screeching muscle cars, a Duesy or two, exotic European racers, electrics, turbos to turbines. Jump up on his century-old farm tractor for Mardi Gras, Hollywood style: The multi-ton steam behemoth turns on car alarms and turns fans out into the streets in pursuit of cell phone images, book signings,and autographs. Gee Whiz and Wow: Heeere’s Jay!