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).

.github/workflows/ci.yml

@@ -80,6 +80,8 @@ jobs:
         run: cargo run --example parameter_api --features derive
       - name: Run async example (async_api_optimization)
         run: cargo run --example async_api_optimization --features async
+      - name: Run visualization example (visualization_demo)
+        run: cargo run --example visualization_demo --features visualization
 
   docs:
     name: Docs