The noise is evaluated at vec3(xz * f, c), so .z is the slope along the held
constant axis. Confirmed numerically (harness/scene/gradAxis.cjs): analytic
.xy matches central differences for fbm and phacelle, .z does not. Only the
ray-march scene's normals were affected; the grid tier already used .xy.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
