Diabetic Retinopathy: 2D Capillary Field & VEGF Diffusion Map

This 2D companion renders the retinal capillary bed as a grid-based reaction-diffusion field instead of an explicit vessel mesh. Every tissue unit runs the same first-order pericyte survival law driven by years of hyperglycemic exposure and HbA1c, and converts to a microaneurysm or a non-perfused (occluded) unit once structural pericyte support drops below threshold — the same staging logic ophthalmologists use to grade NPDR versus PDR. What is genuinely new here: VEGF is solved as a real finite-difference diffusion equation across the grid, so hypoxic regions visibly leak signal into neighbouring tissue, and blood flow is a Darcy pressure relaxation (∇·(k∇P)=0) with occluded units acting as near-zero-permeability barriers, so flow tracers visibly reroute around blocked capillaries rather than simply stopping on a fixed edge.