Summary

Adds native point clustering to the MapLibre ScatterMap trace (#417) via a new Cluster option, exposing plotly.js's scattermap.cluster attributes.

When enabled, nearby markers are grouped client-side (MapLibre's supercluster) into cluster bubbles that progressively expand into individual points as the map is zoomed in — decluttering dense point maps without dropping any data (every point still ships in the trace and becomes individually visible/hoverable at higher zoom).

This is a scattermap-only feature: the deprecated Mapbox-based ScatterMapbox has no equivalent, which is why it's a follow-up rather than part of the field-for-field port in #417.

⚠️ Stacked on #417

Cluster attaches to ScatterMap, so this PR is stacked on #417 → #416 → #410 and should be reviewed/merged after them. Until they land in main, the diff here also shows their commits; once merged, this diff reduces to just the Cluster addition.

What changed

New Cluster struct on plotly::traces::scatter_map, plus a cluster: Option<Cluster> field on ScatterMap (with the usual FieldSetter builder):

• enabled (bool)
• color (color or per-step array, Dim<Box<dyn Color>>)
• maxzoom (0-24; zoom past which clusters expand into points)
• opacity
• size (single or per-step array)
• step (single or per-step array; -1 = auto)

Includes a serialize_cluster unit test and a Cluster doctest.

Testing

• cargo test -p plotly --lib scatter_map — serialize_cluster and serialize_scatter_map pass.
• cargo test -p plotly --doc scatter_map — Cluster and ScatterMap doctests pass.
• cargo clippy — clean.
• cargo +nightly fmt --all -- --check — clean.
• Verified end-to-end rendering in-browser (plotly.js 3.6.0): clusters form at the fitted zoom, expand into individual hoverable points on zoom-in, and maxzoom avoids the MapLibre clusterMaxZoom > source maxzoom warning.

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