Add Choropleth and ChoroplethMap trace types

[jqnatividad]

resolves #408

What's added

Trace types

• Choropleth<Loc, Z> — classic choropleth on the existing geo subplot (LayoutGeo). Includes a LocationMode enum (ISO-3 / USA-states / country names / geojson-id), a dedicated choropleth::Marker (region-boundary line + opacity), Selection/SelectionMarker, z-bounds, and the shared ColorScale/ColorBar machinery. geojson accepts a URL string or an inline GeoJSON object (serde_json::Value).
• ChoroplethMap<Loc, Z> — the modern plotly.js 3.x choroplethmap trace on the MapLibre map subplot (GeoJSON-only).

Layout

• LayoutMap + MapStyle + MapBounds — the MapLibre map subplot, which the crate did not have before (only mapbox). Mirrors the existing Mapbox subplot and is wired into Layout.
• LayoutGeo enhancements for the classic geo subplot:
  • fitbounds (GeoFitBounds: false / locations / geojson) — auto-frames the subplot to the plotted data, overriding center/lonaxis/lataxis when set.
  • resolution (GeoResolution: OneOverOneHundredTenMillion → 110 default, OneOverFiftyMillion → 50) — selects base-layer detail. Per plotly.js, the value is the scale denominator (110 = 1:110,000,000), so 50 gives higher-detail coastlines/land/borders. Pairs naturally with the UN geodata that became the plotly.js v3.1.0 default (bundled plotly.js is 3.6.0).
  • bgcolor — background color of the geo subplot.

Plumbing, tests, docs

• New PlotType::Choropleth / PlotType::ChoroplethMap variants and top-level + sub-module re-exports.
• Serialization tests (exact-JSON), doctests, runnable examples in examples/maps (choropleth + choropleth_map), and a new Maps → Choropleth Maps book recipe page.

Verification

• cargo build --features all; cargo clippy --features all -- -D warnings -A deprecated clean.
• cargo test -p plotly — 315 lib tests + doctests pass (the 8 failing tests are the pre-existing browser-export ones needing chromedriver).
• examples/maps builds and lints clean; cargo +nightly fmt --all -- --check clean in both workspaces.

Notes

• Attribute names/types were taken from the plotly.js v3.6.0 plot-schema.json (traces.choropleth, traces.choroplethmap, layout.map).
• The book recipe embeds the example code (mdBook anchors). maps is intentionally not added to the automated book-example build because its main() performs a network fetch (scatter_geo) and isn't part of CI's BOOK_EXAMPLES.
• Breaking change (new PlotType variants + new layout/trace fields generate new Restyle*/Relayout* enum variants), so this targets the next breaking release (0.15.0), consistent with the other in-flight work.

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