feat: add Treemap and Sunburst trace types

[jqnatividad]

Summary

Adds two hierarchical trace types — Treemap and Sunburst — to the plotly crate, following the existing Pie trace pattern.

Library

• Treemap<V> (plotly/src/traces/treemap.rs): labels/parents/values hierarchy with branch_values, count, level, max_depth, domain, shared Marker, full text/hover field set, plus treemap-specific Tiling (with Packing) and PathBar (with Side) helper structs and a shared BranchValues enum.
• Sunburst<V> (plotly/src/traces/sunburst.rs): same hierarchy/text/hover set plus Leaf (opacity), rotation, and inside_text_orientation.
• New PlotType::Treemap / PlotType::Sunburst variants and top-level re-exports (plotly::Treemap, plotly::Sunburst, and the treemap/sunburst modules).
• New Layout options treemapcolorway / extendtreemapcolors (mirroring the existing sunburst pair).

Tests, examples & docs

• Unit tests + doctests for both traces; layout serialization tests extended to cover the new colorway options.
• Four runnable examples in examples/basic_charts (basic_treemap, styled_treemap, basic_sunburst, styled_sunburst).
• Book pages treemap_charts.md and sunburst_charts.md, linked in SUMMARY.md and the basic-charts overview table.

Verification

• cargo test -p plotly — new unit tests + doctests pass (pre-existing browser static-export tests are unaffected).
• cargo clippy --features ... -- -D warnings clean (lib + examples).
• cargo +nightly fmt --all applied to both workspaces.
• Examples build & run, generating the inline HTML the book embeds.

Notes

• marker.pad/cornerradius/depthfade (treemap-only) are not yet exposed — documented in the Treemap doc comment; the shared Marker covers colors/colorscale/colorbar/line.
• No CHANGELOG.md entry is included (the working-tree edit was reverted intentionally); happy to add one referencing this PR's number.

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