Everyone knows that the under-construction Millennium Tower in Downtown Crossing is the most important thing to happen to Boston since the British split in 1776. The 685-foot spire is set to (maybe) transform the entire neighborhood around it as well as to usher in a Manhattan-like level of condo pricing the likes of which our fair town has never seen before. Yet for all its pomp and circumstance the massive monolith has been shrouded in mystery, at least physically. A new listing, however, for what is known address-wise as 1 Franklin Street, provides us a thorough glance at just what a Millennium Tower spread might entail.