In the long-gestating, career-encompassing allegory that is “Megalopolis,” director Francis Ford Coppola puts his name above the title and, in the film’s lone act of modesty, the words “A Fable” beneath it. To call this garish, idea-bloated monstrosity a mere “fable” is to grossly undersell the project’s expansive insights into art, life and legacy. Here, backed by an estimated $120 million of the “Godfather” director’s own money, is the sort of big swing audiences and critics have come to adore him for: a recklessly ambitious, ginormous epic in which humanity’s eternal themes — greed, corruption, loyalty and power — threaten to suffocate a more intimate personal crisis. In this case, a conservative…