refactor: reduce #[allow] attributes and extract per-distribution helpers

- Remove stale #[allow(dead_code)] and #[allow(too_many_lines)] attributes
- Replace #[allow(dead_code)] with #[cfg(test)] for test-only methods
- Delete unused fill_remaining_independent() and ParamMeta.dist field
- Extract sample_float/int/categorical helpers from TpeSampler, MotpeSampler,
  and SamplingEngine to shorten match-heavy sample() methods
- Extract try_fit_kdes() helper to consolidate repeated fallback blocks
- Refactor sample_tpe_float/int to take &FloatDistribution/&IntDistribution
  instead of destructured args, removing #[allow(too_many_arguments)]

src/study/export.rs

@@ -224,7 +224,7 @@ where
 impl<V: PartialOrd + Clone + serde::Serialize> Study<V> {
     /// Export trials as a pretty-printed JSON array to a file.
     ///
-    /// Each element in the array is a serialized [`crate::CompletedTrial`].
+    /// Each element in the array is a serialized [`CompletedTrial`](crate::sampler::CompletedTrial).
     /// Requires the `serde` feature.
     ///
     /// # Errors