There are still sewing-machine shops and zipper-mongers here, but the real manufacturing lately has been of the real-estate variety. The monumental Hudson Yards overhaul includes two residential towers with over 1,200 combined apartments, not counting what’s to come at the Related Companies’ 12-million-square-foot megacomplex in the mid-Thirties.Then there’s Hudson Crossing, the Townsend, and an unfinished condo by Karl Fischer, all on 37th Street, bringing hundreds more units online.