Function generic_permutation_importance 

pub fn generic_permutation_importance(
    model: &dyn FpcPredictor,
    data: &FdMatrix,
    y: &[f64],
    n_perm: usize,
    seed: u64,
) -> Option<FpcPermutationImportance>

Generic permutation importance for any FPC-based model.

Uses R² for regression, accuracy for classification.