A 52-year-old woman from Mumbai with type 2 diabetes (HbA1c 9.2%) presents with sudden onset of blurred vision in both eyes over 2 weeks. She reports seeing a shadow or curtain-like opacity in the superior visual field that is worse in bright light. On slit-lamp examination, there are granular opacities and vacuoles visible at the posterior lens surface, with the nucleus and cortex appearing relatively clear. What is the most likely type of cataract, and what is the pathophysiological mechanism?