Fancy this: With the wave of a wand, your American dream comes true right before your eyes, move-in ready in just days and at a bargain-basement price. That’s the wish of startups and labs expanding 3-D printing from architectural modeling and parts production to actual home building. …At least one Boston-area company is pursuing that. Prisna, of Everett, is promoting online its “patent-pending automated construction process,’’ transmuting 3-D computer models into “instructions for the robotic fabrication of structures.’’ “The biggest issue is getting cities and towns comfortable with the building-permit process for them, given the materials they’re made of,’’ said Greg Vasil, chief executive of the Greater Boston Real Estate Board. “They’re very different.’’ House-printing companies have to navigate permitting processes not designed for them… Vasil said.