On Saturday night, a singer with a 13-year career played only her second London headline show. Naturally, the venue was packed out with rapt, merchandise-clad fans eager to see her live. But “live”, in this instance, is a loose concept – because Hatsune Miku is a hologram, or virtual idol. Miku’s name comes from a blend of the Japanese words for “first”, “sound” and “future” and the turquoise-hair