A 58-year-old man from Delhi presents with sudden onset of painless vision loss in his right eye. He reports seeing a dark shadow that started from the superior temporal field and progressively moved downward over 2 hours. On examination, visual acuity is 6/9 in the right eye. Dilated fundoscopy reveals a pale, edematous optic disc with retinal whitening in the superior half. Intraocular pressure is 16 mmHg bilaterally. What is the most likely diagnosis?