LIN-MANUEL MIRANDA SLUMPS into a chair, exhausted. For an instant. Then he’s gleefully leaping on top of the chair to demonstrate a Dick Van Dyke dance move. The whiplash moment fits the exhilarating and strange point at which Miranda finds himself: chugging through the meat grinder of international publicity appearances for Mary Poppins Returns while eagerly looking ahead to two other returns—to starring in Hamilton, in Puerto Rico in January, to benefit hurricane-recovery efforts, and to his Washington Heights apartment in February, where he can start writing one of “three big ideas. It’s too early to know what form they will take—stage, film, or TV.” Tomorrow he flies to Mexico City, followed by Paris, then London, and finally Miami. For now, though, Miranda is fully, happily present in this Central…