Kenneth Branagh has found a nice rhythm in his Hercule Poirot mysteries, serving as director, producer and star. “A Haunting in Venice,” his third go-round with writer Michael Green, derives its main wrinkle from the vaguely supernatural tone, a just-in-time-for-Halloween adornment that might add a small commercial boost to a formula otherwise as neatly manicured as the great detective’s moustache.