The architect David Adjaye is realizing a dream project. At the end of this month, he will open the National Museum of African American History and Culture — the 12th and final Smithsonian museum on the National Mall — on five acres that were once home to a slave market, just beyond the White House. Personal experience, as well as historical memory, informs Adjaye’s architectural decisions. Throughout the museum, he strives for what he calls an “ease of navigation” — an idea he thinks about a lot.