Thanks to the 1964 movie, "Mary Poppins" is the kind of musical where you find yourself humming the songs on the way into the theater, then greet their teasing quotation in the overture as a guarantee of musical comedy pleasure to come. That guarantee is as good as gold. This stage version, already a hit in London, arrived on Broadway last night in a production both lavish and loving, never content just to refer us back to the movie but determined to whip up a supercalifragilisticexpialidocious entertainment of its own. And yes, that is a word: My spell check recognizes it.