Adds cultmath_smin_grad, the CultCellular struct, and cultmath_cellular to
both tracked copies of CultMath.hlsl (packages/cultmath/shaders and the Unity
package), placed after cultmath_pcg4d so cultmath_cellular can call
cultmath_pcg3d and cultmath_distance, both of which HLSL requires to be
declared earlier in the file. Both copies stay byte-identical.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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