A 68-year-old Indian man presents to the ophthalmology clinic with progressive painless blurring of vision in both eyes over 3 years. He reports difficulty reading newspapers and watching television. On examination, visual acuity is 6/24 in both eyes. Slit-lamp examination reveals a brown-coloured opacity in the nucleus of both lenses with a characteristic radial brown discolouration. The patient denies trauma, prolonged corticosteroid use, or metabolic disease. What is the most likely diagnosis?