A 72-year-old woman from Bangalore presents with sudden onset of a dark shadow in the upper half of her left visual field. She denies eye pain or diplopia. On examination, visual acuity is 6/6 in both eyes. Pupils are equal and reactive. Extraocular movements are intact. Perimetry shows a left superior altitudinal visual field defect. Fundoscopy reveals a pale, swollen optic disc with superficial flame-shaped hemorrhages at the superior margin. What is the most likely diagnosis?