feat(pareto): add non_dominated_sort

Deb's fast non-dominated sort: returns Vec<Vec<usize>> of front indices
into the input population, with fronts[0] being the non-dominated set.
O(N²·M) is acceptable for v1 (spec §9.5).

Tests cover: small known population produces expected fronts; equal
candidates land on the same front; an empty population yields no
fronts.

src/prelude.rs

@@ -11,4 +11,6 @@ pub use crate::core::{
 
 pub use crate::traits::{Initializer, Optimizer, Variation};
 
-pub use crate::pareto::{Dominance, best_candidate, pareto_compare, pareto_front};
+pub use crate::pareto::{
+    Dominance, best_candidate, non_dominated_sort, pareto_compare, pareto_front,
+};