Port material-color-utilities' color pipeline to Nix: sRGB<->CAM16<->HCT with
the full HctSolver (findResultByJ Newton step + bisectToLimit over the 255
critical planes). hctFromHex/hexFromHct round-trip bit-exact across a 125-color
grid, including saturated colors that hit the bisect fallback. Constants ported
from the canonical TypeScript reference.
Brick 2 of the pure-Nix matugen effort. Standalone lib, not yet wired in.
Assisted-by: pi (opus-4.8)
