smoothing_iterations used explicit cotangent diffusion `f += 0.5 * Minv @ L @ f`,
which is unstable: the Laplace-Beltrami spectral radius scales like 1/edge^2, so a
fixed step diverges on fine meshes (and obtuse triangles add negative cotangent
weights) -> fields exploded to ~1e12. Replaced with row-normalized umbrella
neighbour averaging (a convex combination), unconditionally stable at any
resolution. Verified: on a fine noisy sphere, smoothing now monotonically reduces
the field variance with finite values instead of blowing up.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
