Tall and gangly, with a cleanly shaven head and an unearthly wail that could shatter glass, the Skunk Anansie frontwoman known as Skin is the kind of indie rock unicorn who comes along once a in blue moon. It’s been 15 years since she and the band first played in the States, and as headliners of this year’s Afropunk festival they join the ranks of O.G. rapper Ice Cube and legendary punk outfit Bad Brains, musicians who conjured the rebellious spirit of Afropunk eons before the idea even existed. And yet for someone who has been cultivating fierceness for over two decades, the British singer has a remarkably soft speaking voice. “I must confess, I’m a little bit distracted,” she whispers apologetically over the phone from her home in London. “I just got inside information that Björk tickets are about to go on sale any minute now.” The line goes dead for about 60 seconds as she fumbles for her credit card . . . then, a guttural roar.  “Yessss! My order has been processed!”