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/operators/composite.rs

@@ -0,0 +1,96 @@
+//! Compose two `Variation` operators into a pipeline (typically crossover → mutation).
+
+use crate::core::rng::Rng;
+use crate::traits::Variation;
+
+/// A two-stage variation pipeline.
+///
+/// On each call to `vary`:
+///
+/// 1. The `crossover` operator is run on the input `parents`, producing one
+///    or more children.
+/// 2. For every child, the `mutation` operator is run with that child as its
+///    sole parent, and the resulting children are concatenated into the
+///    output.
+///
+/// Use this to build the canonical NSGA-II operator stack — SBX followed by
+/// polynomial mutation — out of the existing primitives:
+///
+/// ```rust
+/// use heuropt::prelude::*;
+///
+/// let bounds = vec![(0.0, 1.0); 30];
+/// let variation = CompositeVariation {
+///     crossover: SimulatedBinaryCrossover::new(bounds.clone(), 15.0, 0.5),
+///     mutation: PolynomialMutation::new(bounds, 20.0, 1.0 / 30.0),
+/// };
+/// let _ = variation;
+/// ```
+#[derive(Debug, Clone)]
+pub struct CompositeVariation<C, M> {
+    /// First-stage operator; typically a crossover that consumes ≥ 2 parents.
+    pub crossover: C,
+    /// Second-stage operator; typically a mutation that consumes 1 parent.
+    pub mutation: M,
+}
+
+impl<D, C, M> Variation<D> for CompositeVariation<C, M>
+where
+    D: Clone,
+    C: Variation<D>,
+    M: Variation<D>,
+{
+    fn vary(&mut self, parents: &[D], rng: &mut Rng) -> Vec<D> {
+        let crossed = self.crossover.vary(parents, rng);
+        let mut out = Vec::with_capacity(crossed.len());
+        for child in crossed {
+            let mutated = self.mutation.vary(std::slice::from_ref(&child), rng);
+            out.extend(mutated);
+        }
+        out
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::core::rng::rng_from_seed;
+    use crate::operators::real::{
+        BoundedGaussianMutation, PolynomialMutation, SimulatedBinaryCrossover,
+    };
+
+    #[test]
+    fn pipes_sbx_into_polynomial_mutation() {
+        let bounds = vec![(-1.0, 1.0); 4];
+        let mut variation = CompositeVariation {
+            crossover: SimulatedBinaryCrossover::new(bounds.clone(), 15.0, 1.0),
+            mutation: PolynomialMutation::new(bounds, 20.0, 0.25),
+        };
+        let mut rng = rng_from_seed(123);
+        let p1 = vec![0.1, -0.2, 0.3, -0.4];
+        let p2 = vec![-0.3, 0.4, -0.1, 0.2];
+        let children = variation.vary(&[p1, p2], &mut rng);
+        // SBX produces 2 children; polynomial mutation produces 1 child each.
+        assert_eq!(children.len(), 2);
+        for c in &children {
+            assert_eq!(c.len(), 4);
+            for &x in c {
+                assert!(x >= -1.0 && x <= 1.0);
+            }
+        }
+    }
+
+    #[test]
+    fn output_count_equals_inner_crossover_count_when_mutation_is_1to1() {
+        // BoundedGaussianMutation always returns 1 child.
+        let bounds = vec![(0.0, 1.0); 3];
+        let mut variation = CompositeVariation {
+            crossover: SimulatedBinaryCrossover::new(bounds.clone(), 10.0, 0.5),
+            mutation: BoundedGaussianMutation::new(0.05, bounds),
+        };
+        let mut rng = rng_from_seed(0);
+        let parents = vec![vec![0.5, 0.5, 0.5], vec![0.25, 0.75, 0.5]];
+        let children = variation.vary(&parents, &mut rng);
+        assert_eq!(children.len(), 2);
+    }
+}

src/operators/mod.rs

@@ -1,9 +1,11 @@
 //! Built-in operators for common decision types.
 
 pub mod binary;
+pub mod composite;
 pub mod permutation;
 pub mod real;
 
 pub use binary::*;
+pub use composite::*;
 pub use permutation::*;
 pub use real::*;

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::{