Neural Crest Migration — Continuum Density Field

This is a 2D continuum counterpart to the 3D neural-crest agent simulation: instead of individually tracked cells, a cell-density field is solved directly on a grid, with chemotaxis represented as advection up a fixed SDF1 gradient, contact inhibition of locomotion represented as a density-dependent (crowding-activated) diffusion term, and co-attraction represented as an aggregation flux toward locally denser regions. The same three local rules from the original mean-field literature on neural crest streaming are recovered as terms in a single conservation law, solved with an explicit finite-volume scheme, giving an independent, mathematically equivalent way to see the same collective-migration phenomenon: a cohesive, directed stream that only holds together when chemotaxis, CIL and co-attraction are balanced.