This complements ChoroplethMap support (#410) by adding density-heatmap trace for it. plotly.js provides densitymap as the MapLibre-based successor to the (Mapbox-based) densitymapbox, which is deprecated upstream. We should add a matching DensityMap Rust trace.

Motivation

DensityMapbox targets the older Mapbox subplot (layout.mapbox). New work should be able to draw weighted density heatmaps on the same layout.map subplot that ChoroplethMap already uses, with full color-scale and hover support.

Proposed change

Add a DensityMap<Lat, Lon, Z> trace that mirrors ChoroplethMap's structure but with a density data model:

• Data: lat, lon, z (per-point weight), radius, opacity
• Color: zauto, zmin/zmax/zmid, colorscale, autocolorscale, reversescale, showscale, colorbar, coloraxis
• Hover: text, hovertext, hoverinfo, hovertemplate, hovertemplatefallback, hoverlabel
• Map/misc: subplot (defaults to "map"), below, ids, meta, customdata, uirevision

No marker/selected/unselected (density traces have none). Registration: a DensityMap variant on PlotType (serializes to "densitymap"), plus module/type re-exports.

Dependency

This builds on the MapLibre map subplot (LayoutMap/MapStyle) introduced in #410, so the PR is stacked on that work.