Last spring, Mark W. Grapin promised his two sons he would build them a treehouse in the maple tree beside their Fairfax County home before he shipped out to Iraq again with the Army National Guard. But now that he’s run afoul of county zoning laws, Grapin has given his sons a civics lesson, too. Grapin spent about $1,400 in materials to build the treehouse in a maple beside his Falls Church home.