snoise_grad.w is bit-equal to snoise (stronger than the spec's 1e-6
floor, measured over 20,000 points). fbm_grad/ridged_grad with 1/3
octaves are pinned bit-exact against a closed form built with the same
incremental gain/lacunarity multiply chain the production loop uses,
catching both a swapped-argument mutant and an applied-once mutant.
The shared invariant-8 property harness central-differences all three
functions' own .w, filtering simplex-lattice seam flukes by comparing
the estimate at two step sizes (measured tolerances and detection
floor in the harness comment). A dedicated fixture bisects to a snoise
zero crossing and pins that ridged_grad's two one-sided gradients there
have opposite sign.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
bbf00394de