WebGPU Marching Cubes

Dynamic marching-cubes mesh generation, ported from Thibault Coppex's WebGPU reference as a multi-file TinyFX example.

A GPU append list classifies a 48-cubed volume from TinyFX sdf:: primitives, then its GPU-owned triangle count drives an indirect draw. The vertex stage reconstructs interpolated surface vertices and normals; Poly Haven rock, ground, and mud assets provide triplanar color, normal, and height detail.