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/pareto.rs

@@ -29,8 +29,8 @@
 //!
 //! Internally, this module also provides the fast non-dominated sorting
 //! algorithm (Deb et al., 2002) used by
-//! [`MultiObjectiveStudy::pareto_front()`](crate::MultiObjectiveStudy::pareto_front)
-//! and [`Nsga2Sampler`](crate::Nsga2Sampler).
+//! [`MultiObjectiveStudy::pareto_front()`](crate::multi_objective::MultiObjectiveStudy::pareto_front)
+//! and [`Nsga2Sampler`](crate::sampler::Nsga2Sampler).
 //!
 //! # Example
 //!