feat: add CMA-ES sampler behind cma-es feature flag

Implement CMA-ES (Covariance Matrix Adaptation Evolution Strategy) as a
new sampler for continuous optimization. Uses nalgebra for matrix
operations and implements the full Hansen 2016 algorithm: mean update,
evolution paths, covariance matrix adaptation, and cumulative step-size
adaptation.

Key features:
- Builder API with configurable sigma0, population_size, and seed
- Three-phase state machine: discovery, active sampling, generation update
- Rejection sampling with bounds clipping fallback
- Log-scale and step parameter support via internal-space mapping
- Categorical parameters handled via random sampling fallback
- Numerical robustness: eigenvalue clamping, symmetry enforcement

src/sampler/mod.rs

@@ -1,5 +1,7 @@
 //! Sampler trait and implementations for parameter sampling.
 
+#[cfg(feature = "cma-es")]
+pub mod cma_es;
 pub mod grid;
 pub mod random;
 #[cfg(feature = "sobol")]