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/parameter.rs

@@ -46,7 +46,7 @@ use crate::distribution::{
     CategoricalDistribution, Distribution, FloatDistribution, IntDistribution,
 };
 use crate::error::{Error, Result};
-use crate::param::ParamValue;
+pub use crate::param::ParamValue;
 use crate::trial::Trial;
 
 static NEXT_PARAM_ID: AtomicU64 = AtomicU64::new(0);