Dmitri Horostovsky, a singer possessed of a penetrating baritone as deep as his Siberian soul, along with matinee-idol looks that made him a star beyond the classical-music world, died Wednesday. He was 55 and had waged a heart-breaking battle with brain cancer since revealing his illness in June 2015. His management agency, Askonas Holt, said he died in hospice care near his London home. To roles as diverse as Verdi’s tragic court jester Rigoletto, Mozart’s faithless Don…