feat(operators): add CompositeVariation pipeline (crossover → mutation)

Generic two-stage Variation operator: runs an inner crossover-style
operator on the parents, then applies an inner mutation-style operator
to each resulting child. Lets users build the canonical NSGA-II
operator stack — `SimulatedBinaryCrossover` followed by
`PolynomialMutation` — by composing the existing primitives instead
of bundling a one-off SbxPolyMut struct.

Lives in src/operators/composite.rs to keep type-specific operator
files unchanged. Generic over decision type and over both inner
operators.

src/prelude.rs

@@ -17,8 +17,8 @@ pub use crate::pareto::{
 };
 
 pub use crate::operators::{
-    BitFlipMutation, BoundedGaussianMutation, GaussianMutation, PolynomialMutation,
-    RealBounds, SimulatedBinaryCrossover, SwapMutation,
+    BitFlipMutation, BoundedGaussianMutation, CompositeVariation, GaussianMutation,
+    PolynomialMutation, RealBounds, SimulatedBinaryCrossover, SwapMutation,
 };
 
 pub use crate::algorithms::{