TOKYO — Japan’s Constitution promises gender equality, and Prime Minister Shinzo Abe says he wants a society where all women can "shine.” But many women say that is hard to do when they cannot even use their own surnames. Under a Japanese law that dates back to the Meiji era, more than a century ago, all married couples must use one surname. In theory, a couple may choose either the husband’s or the wife’s last name, but in practice, 96 percent of women assume their husband’s.