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/study.rs

@@ -2154,6 +2154,22 @@ impl Study<f64> {
     }
 }
 
+#[cfg(feature = "visualization")]
+impl Study<f64> {
+    /// Generates an HTML report with interactive Plotly.js charts.
+    ///
+    /// Creates a self-contained HTML file that can be opened in any browser.
+    /// See [`generate_html_report`](crate::visualization::generate_html_report)
+    /// for details on the included charts.
+    ///
+    /// # Errors
+    ///
+    /// Returns an I/O error if the file cannot be created or written.
+    pub fn export_html(&self, path: impl AsRef<std::path::Path>) -> std::io::Result<()> {
+        crate::visualization::generate_html_report(self, path)
+    }
+}
+
 /// A serializable snapshot of a study's state.
 ///
 /// Since [`Study`] contains non-serializable fields (samplers, atomics, etc.),