While the film’s plot progresses, with a few divagations, in a straight line through the decades of Benjamin Button’s life, the backward vector of that biography turns this “Curious Case” into a genuine mystery. And the puzzles it invites us to contemplate — in consistently interesting, if not always dramatically satisfying ways — are deep and imposing, concerning the passage of time, the elusiveness of experience and the Janus-faced nature of love.