Dimension Rigidity and Projective Geometry of Trace-Product Switchings of the Gold Cube

We completely classify a natural scalar trace-product switching of the Gold almost perfect nonlinear function x x cubed in every even dimension. Nontrivial switchings occur only for n=4,6,8: the admissible coefficients are, respectively, the nonzero trace-zero elements, the six elements of multiplicative order nine, and F 4*. For every even n at least10, no nonzero coefficient is admissible. The infinite range is excluded by additive-character estimates on a Fermat cubic, with exact finite bridges for n=10,12. The raw coefficient lists for n=6,8 appeared earlier in Arshad's dissertation; our contribution is their intrinsic description, a proof uniform in the dimension, and the resulting dimension-rigidity theorem. We also classify normalized rank-two extensions in dimension eight by P 1(F 4). A binary trace selector accepts two coefficient values at each non-base projective point, and the eight accepted marked switchings form exactly two extended-affine, hence two CCZ, classes. A centre-independent low-rank derivative criterion reduces each rank- r candidate to 2 r-1 membership tests in precomputed forbidden sets. The global APN classes reached are known; the results describe their local organization around the Gold centre and rule out this switching mechanism in all larger even dimensions.