Stryker (scoped to this cut's diff) survived two mutants on the grad2 ternary
in math.cellular: forcing the true branch unconditionally, and loosening
f2 > 0.0f to f2 >= 0.0f. Both are real findings about the code, not the
tests: F2 = 0 needs two distinct integer cells' pcg3d-jittered feature points
to land on the exact float32 value in every component, which the jitter's
~24-bit float precision makes unreachable for any p this function is
actually called with. The spec only asks for the degenerate guard at F1 = 0;
F2's copy of it was added for symmetry and had no reachable case to defend.
Deleted the guard (grad2 is now a bare (p - c2) / f2) in math.cs and both
HLSL mirrors, and corrected design.md's invariant-8 section, which had
overstated this as symmetric with F1's guard.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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