Synthesizes a radial gradient PNG fading from <centerHex> at the
image center to <edgeHex> at the corner. Distance is normalized so
the corner is t=1 (works for non-square images), with a smoothstep
curve giving a soft falloff rather than a harsh disc edge.
Useful for spell glow rings, vignettes, soft-edged decals, and
particle-emitter masks — the common "circular blob" cases that
linear gradients can't produce.
Verified: 64×64 white-center / black-edge "glow" PNG written
successfully; invalid 'notacolor' hex rejected with exit 1.
Brings command count to 222.
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