feat: add BOHB sampler for budget-aware Bayesian optimization

BOHB combines TPE's model-guided sampling with Hyperband's budget-aware
evaluation by conditioning the TPE model on trials at specific budget
levels, producing better-calibrated parameter proposals.

src/sampler/mod.rs

@@ -1,5 +1,6 @@
 //! Sampler trait and implementations for parameter sampling.
 
+pub mod bohb;
 #[cfg(feature = "cma-es")]
 pub mod cma_es;
 pub mod grid;