Constant POLY_COMPANION_CLOSED_DEG 

pub const POLY_COMPANION_CLOSED_DEG: usize = 2; // 2usize

Highest degree whose companion eigenvalues have a closed form on this type.

Degree 1 is linear. Degree 2 is the characteristic polynomial of the 2×2 companion (quadratic formula). Higher-degree companions are not symmetric, so ExactNumArray::eigen_decomp cannot be used; those degrees return None from ExactNumPoly::roots_real.