feat: add Violin trace type

[jqnatividad]

Summary

resolves #411

Adds the Violin trace type, matching the full attribute surface of plotly.js's violin trace.

Violin plots visualize the distribution of numerical data using a kernel density estimate, optionally with an inner box plot, mean line, and sample points.

What's included

• Violin<X, Y> trace with all violin-specific attributes: bandwidth, box (inner mini box plot), meanline, span/spanmode, scalegroup/scalemode, side (split violins), points/jitter/pointpos, selected/unselected, quartilemethod, hoveron, zorder, x0/y0, plus the standard base-trace attributes.
• Supporting types: ViolinBox, MeanLine, Selection/SelectionMarker, and enums ViolinPoints, ScaleMode, SpanMode, ViolinSide, HoverOn (reuses box_plot::QuartileMethod).
• Registration: PlotType::Violin, module + re-exports in traces/mod.rs and lib.rs. Grouped/split violins work via the existing ViolinMode layout attribute.
• Tests: 12 unit tests + a doctest.
• Example: statistical_charts gains basic, horizontal, and split violin examples.
• Book: new Violin Plots recipe under Statistical Charts.

Attribute names were verified against plotly.js src/traces/violin/attributes.js.

Checklist

• cargo test -p plotly passes (12 violin unit tests + doctest)
• cargo clippy --features all -- -D warnings -A deprecated clean
• cargo +nightly fmt --all
• CHANGELOG.md updated

🤖 Generated with Claude Code

[jqnatividad]

See violin plots in action using this PR as a git dependency.
https://dathere.github.io/qsv/gallery.html

[andrei-ng]

Thank you for the PRs @jqnatividad!

I will review them in the next couple of days. I do have one request though, can you give me rights to update the PR branches? I would prefer to rebase them locally before merging but I cannot push to your fork.
There is an option in the PR settings when you create them to allow me to update the PR's branch. I think it is under the comments and is called "Allow edits by maintainers"

[jqnatividad]

Thanks @andrei-ng! Sent you a write-access collaborator invite on the dathere/plotly fork — once accepted you can rebase/push all the branches. Also flipped on "Allow edits by maintainers" for the open PRs (#410, #416, #417, #418). Let me know if anything still blocks you.

Since the fork is org-owned, the maintainer-edits checkbox can be hit-or-miss, so the collaborator invite is the reliable route.

[andrei-ng]

Thanks @andrei-ng! Sent you a write-access collaborator invite on the dathere/plotly fork — once accepted you can rebase/push all the branches. Also flipped on "Allow edits by maintainers" for the open PRs (#410, #416, #417, #418). Let me know if anything still blocks you.

Since the fork is org-owned, the maintainer-edits checkbox can be hit-or-miss, so the collaborator invite is the reliable route.

Awesome! Thanks!