feat: add HTML visualization reports with Plotly.js charts

Add a `visualization` feature flag that generates self-contained HTML
reports with interactive Plotly.js charts for offline visualization of
optimization results. Charts include optimization history, slice plots,
parallel coordinates, parameter importance, trial timeline, and
intermediate values (learning curves).

src/lib.rs

@@ -190,6 +190,7 @@
 //! - `serde`: Enable `Serialize`/`Deserialize` on public types and `Study::save()`/`Study::load()`
 //! - `sobol`: Enable the Sobol quasi-random sampler for better space coverage
 //! - `cma-es`: Enable the CMA-ES sampler for continuous optimization
+//! - `visualization`: Generate self-contained HTML reports with interactive Plotly.js charts
 //! - `tracing`: Emit structured log events via the [`tracing`](https://docs.rs/tracing) crate at key optimization points
 
 /// Emit a `tracing::info!` event when the `tracing` feature is enabled.
@@ -229,6 +230,8 @@ pub mod sampler;
 mod study;
 mod trial;
 mod types;
+#[cfg(feature = "visualization")]
+mod visualization;
 
 pub use error::{Error, Result, TrialPruned};
 pub use multi_objective::{MultiObjectiveSampler, MultiObjectiveStudy, MultiObjectiveTrial};
@@ -258,6 +261,8 @@ pub use study::Study;
 pub use study::StudySnapshot;
 pub use trial::{AttrValue, Trial};
 pub use types::{Direction, TrialState};
+#[cfg(feature = "visualization")]
+pub use visualization::generate_html_report;
 
 /// Convenient wildcard import for the most common types.
 ///
@@ -294,4 +299,6 @@ pub mod prelude {
     pub use crate::study::StudySnapshot;
     pub use crate::trial::{AttrValue, Trial};
     pub use crate::types::Direction;
+    #[cfg(feature = "visualization")]
+    pub use crate::visualization::generate_html_report;
 }