feat(operators): add LevyMutation real-valued heavy-tailed mutation

Lévy-flight perturbation: each variable receives a step drawn from a
heavy-tailed Lévy(α) distribution rather than a Normal. The result is
"mostly small steps with rare big jumps," which gives a more
exploratory mutation than Gaussian without abandoning local search.

Decision type: Vec<f64>, with optional bounds (clamped per-axis if
`bounds` is non-empty). The step is sampled via Mantegna's algorithm
which generates Lévy(α) by combining two Normal samples and taking
the right power, controlled by the tail exponent `alpha` (typical
1.5; 1 is heavy, 2 collapses to Normal).

This is the only genuinely-different mutation kernel from Cuckoo
Search and other Lévy-flight metaheuristics; ship it as a Variation
operator usable from any algorithm rather than as a separate
algorithm.

src/prelude.rs

@@ -18,7 +18,7 @@ pub use crate::pareto::{
 
 pub use crate::operators::{
     BitFlipMutation, BoundedGaussianMutation, CompositeVariation, GaussianMutation,
-    PolynomialMutation, RealBounds, SimulatedBinaryCrossover, SwapMutation,
+    LevyMutation, PolynomialMutation, RealBounds, SimulatedBinaryCrossover, SwapMutation,
 };
 
 pub use crate::algorithms::{