feat(operators): add SwapMutation for permutations

Variation that clones the first parent (a Vec<usize> permutation) and
swaps two distinct random indices when len >= 2 (spec §11.4). Tests
confirm the multiset of contents is preserved.

src/prelude.rs

@@ -16,4 +16,4 @@ pub use crate::pareto::{
     pareto_compare, pareto_front,
 };
 
-pub use crate::operators::{BitFlipMutation, GaussianMutation, RealBounds};
+pub use crate::operators::{BitFlipMutation, GaussianMutation, RealBounds, SwapMutation};