nx_colorsci_cat_gate.nx

nx_colorsci_cat_gate.nx -- THE GATE FOR CHROMATIC ADAPTATION in the colour-science ruler, 2026-09-03. WHY THIS EXISTS. The ICC PCS is D50 by the spec and real captures are made under other illuminants, most often D65. The profile path in nx_colorsci_lib applies matrix plus TRC only, so before this rung a colour measured under a non-PCS white carried a white-point error into every downstream gamut and CIEDE2000 judgement. The error never announced itself: every number stayed in range and merely read slightly wrong. cs_cat_* closes that, and this gate is what makes the closure evidence rather than a claim. SUBJECT: cs_cat_cone, cs_cat_build, cs_cat_apply and cs_cat_xyz in-process (the lib has no main). Teeth, in order: T1 Bradford D65 to D50 builds at all. T2 KAT: all NINE coefficients against the published Bradford D65-to-D50 matrix. A mistyped cone matrix or a transposed multiply cannot pass nine independent coefficients. T3 THE DEFINING INVARIANT: the transform carries the SOURCE white exactly onto the DESTINATION white. This one needs no external table -- it is what chromatic adaptation MEANS, so it holds even if every published digit above were mistyped, and the two teeth therefore fail independently. T4 adapting a white to ITSELF is the identity matrix (the diagonal gain degenerates to one). T5 round trip: D65 to D50 and back returns the probe. T6 ANTI-VACUITY NEG-CONTROL: CAT02 must NOT equal Bradford. Without this a cs_cat_cone that ignored its argument and always wrote Bradford would pass every other tooth on this page. T7 the invariant of T3 holds for CAT02 too, so T6 cannot pass by CAT02 being merely wrong. T8 NEG-CONTROL: an unknown transform id REFUSES (returns 0) instead of defaulting to Bradford. T9 NEG-CONTROL: a zero source white REFUSES instead of dividing by zero or emitting an identity. Tolerance is the half-ulp of the published seven-decimal matrix widened for Q30 rounding; the coefficients of the two transforms differ by ~1e-2, two orders above it, so T6 cannot pass on rounding. license_tier: ORIGINAL No hw writes (Rule 26). expect_exit: 0