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/differential_evolution_tests.rs

@@ -1,7 +1,5 @@
 use optimizer::prelude::*;
-use optimizer::sampler::differential_evolution::{
-    DifferentialEvolutionSampler, DifferentialEvolutionStrategy,
-};
+use optimizer::sampler::de::{DifferentialEvolutionSampler, DifferentialEvolutionStrategy};
 
 #[test]
 fn sphere_function() {

tests/integration.rs

@@ -1707,7 +1707,7 @@ fn test_ask_and_tell_with_custom_value_type() {
 
 use std::collections::HashMap;
 
-use optimizer::ParamValue;
+use optimizer::parameter::ParamValue;
 
 #[test]
 fn test_enqueue_params_evaluated_first() {
@@ -2291,7 +2291,7 @@ fn test_builder_with_sampler() {
 
 #[test]
 fn test_builder_with_pruner() {
-    use optimizer::NopPruner;
+    use optimizer::pruner::NopPruner;
 
     let study: Study<f64> = Study::builder().pruner(NopPruner).build();
 
@@ -2303,7 +2303,7 @@ fn test_builder_chaining() {
     let study: Study<f64> = Study::builder()
         .maximize()
         .sampler(RandomSampler::with_seed(42))
-        .pruner(optimizer::NopPruner)
+        .pruner(optimizer::pruner::NopPruner)
         .build();
 
     assert_eq!(study.direction(), Direction::Maximize);

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)

tests/serde_tests.rs

@@ -2,9 +2,9 @@
 
 use std::collections::HashMap;
 
-use optimizer::parameter::{FloatParam, IntParam, Parameter};
+use optimizer::parameter::{FloatParam, IntParam, ParamValue, Parameter};
 use optimizer::sampler::CompletedTrial;
-use optimizer::{Direction, ParamValue, Study, StudySnapshot, TrialState};
+use optimizer::{Direction, Study, StudySnapshot, TrialState};
 
 #[test]
 fn round_trip_save_load() {