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() {