Rightly cherished for his iconic rendition of “The Star Spangled Banner,” Jimi Hendrix’s appearance at the 1969 Woodstock Festival is an enigmatic treasure. Aside from a dip into his classics (“Fire,” “Purple Haze”), laced with enough spiteful guitar runs to suggest a certain hostility to expectations, there are blues jams in which Hendrix tests his new band (“Villanova Junction”). Most inscrutable is the solo, furiously questing “Woodstock Improvisation,” in which he takes his guitar out for an early-morning spin into everywhere at once.