2D ASMR Sand Cutting: Cohesion & Moisture Granular Model

This is the 2D, top-down companion to the 3D ASMR sand-cutting sim. Instead of orbiting a rendered mesh, it carves a real depth grid directly beneath the cursor and shades it like the tray really looks from above: a bright, undisturbed surface and a darker, shadowed groove wherever the blade has passed. The same radial-influence dig kernel from the 3D version drives the cut, but here cohesion and moisture content are real, adjustable parameters — push cohesion low or moisture to either extreme and the cut walls avalanche and spray grains instead of holding a clean edge, exactly the difference between a satisfying ASMR cut and sand that just won't cooperate.