Widmanstätten Pattern 2D: Diffusion-Front Cross-Section

This is a 2D companion to the 3D Widmanstätten pattern simulator, built around a genuinely different, deeper piece of physics: instead of applying the empirical Wood (1964) spacing formula directly, it numerically solves the diffusion-controlled moving-boundary problem (a Stefan problem) that formula actually comes from. A finite-difference solver integrates Fick's second law for the Ni concentration field ahead of each growing kamacite interface, and the interface itself only advances when the Stefan mass-balance condition at its boundary is satisfied — so the polished-section micrograph and the concentration-profile strip beneath it are the literal output of that integration, frame by frame, as the parent asteroid core cools from a molten 900 °C down toward the ~400 °C point where diffusion becomes too slow to move nickel any further.