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

tests/multi_objective_tests.rs

@@ -1,11 +1,12 @@
 //! Integration tests for multi-objective optimization.
 
+use optimizer::Direction;
 use optimizer::multi_objective::MultiObjectiveStudy;
 use optimizer::parameter::{CategoricalParam, FloatParam, Parameter};
+use optimizer::sampler::Decomposition;
 use optimizer::sampler::moead::MoeadSampler;
 use optimizer::sampler::nsga2::Nsga2Sampler;
 use optimizer::sampler::nsga3::Nsga3Sampler;
-use optimizer::{Decomposition, Direction};
 
 // ---------------------------------------------------------------------------
 // Pareto utility tests (via public MultiObjectiveStudy)