/// How the penalized Hessian's log-determinant and derivatives treat the
/// spectrum below the stability floor.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub enum PseudoLogdetMode {
/// Keep every eigenpair in the smooth spectral regularizer.
#[default]
Smooth,
/// Exclude numerical null-space directions consistently from pseudo-logdet
/// and derivative traces.
HardPseudo,
}