refactor: move all type re-exports out of crate root into their modules

- Sampler, pruner, storage, parameter, and multi_objective types are no
  longer re-exported at the crate root; access via module paths instead
  (e.g. optimizer::sampler::TpeSampler, optimizer::parameter::FloatParam)
- Prelude continues to re-export everything for convenience
- Add module-level pub use re-exports in sampler/mod.rs and parameter.rs
- Update derive macro to reference optimizer::parameter::Categorical
- Rename sampler::differential_evolution to sampler::de
- Fix all downstream imports in tests, benches, and doctests
- Fix all rustdoc intra-doc links to use explicit module paths

src/sampler/mod.rs

@@ -3,7 +3,7 @@
 pub mod bohb;
 #[cfg(feature = "cma-es")]
 pub mod cma_es;
-pub mod differential_evolution;
+pub mod de;
 pub(crate) mod genetic;
 #[cfg(feature = "gp")]
 pub mod gp;
@@ -19,6 +19,22 @@ pub mod tpe;
 
 use std::collections::HashMap;
 
+pub use bohb::BohbSampler;
+#[cfg(feature = "cma-es")]
+pub use cma_es::CmaEsSampler;
+pub use de::{DifferentialEvolutionSampler, DifferentialEvolutionStrategy};
+#[cfg(feature = "gp")]
+pub use gp::GpSampler;
+pub use grid::GridSearchSampler;
+pub use moead::{Decomposition, MoeadSampler};
+pub use motpe::MotpeSampler;
+pub use nsga2::Nsga2Sampler;
+pub use nsga3::Nsga3Sampler;
+pub use random::RandomSampler;
+#[cfg(feature = "sobol")]
+pub use sobol::SobolSampler;
+pub use tpe::TpeSampler;
+
 use crate::distribution::Distribution;
 use crate::param::ParamValue;
 use crate::parameter::{ParamId, Parameter};