refactor(sampler): extract shared utilities to reduce duplication

- Add sampler/common.rs with distribution helpers (internal_bounds,
  from_internal, to_internal, sample_random) used by 8 samplers
- Add sampler/tpe/common.rs with TPE sampling functions
  (sample_tpe_float, sample_tpe_int, sample_tpe_categorical)
  shared by TpeSampler, MultivariateTpeSampler, and MotpeSampler
- Remove ~940 lines of near-identical code across sampler modules

src/sampler/mod.rs

@@ -3,6 +3,7 @@
 pub mod bohb;
 #[cfg(feature = "cma-es")]
 pub mod cma_es;
+pub(crate) mod common;
 pub mod de;
 pub(crate) mod genetic;
 #[cfg(feature = "gp")]