fn symmetric_eigen(a: &[f64], n: usize) -> (Vec<f64>, Vec<f64>)Expand description
Descending eigenvalues and sign-canonicalised eigenvectors of a symmetric
matrix a (row-major n × n) via nalgebra’s symmetric eigensolver
(tridiagonalisation + implicit QR — LAPACK-grade, replacing a hand-rolled
Jacobi sweep). Each eigenvector column is sign-fixed so its largest-magnitude
entry is positive, making downstream projections deterministic across
backends. Eigenvectors are returned row-major (vecs[row * n + col]).