feat: expose SuperCluster properties on clusters

[hadt0786]

Summary

Expose SuperCluster aggregated properties on the resulting Cluster object so custom renderers can access values produced by SuperCluster map / reduce functions.

Changes

• add an optional properties field to Cluster
• preserve SuperCluster feature properties when converting a SuperCluster result into a Cluster
• apply the same behavior to SuperClusterViewportAlgorithm
• add regression coverage confirming aggregated properties are available through cluster.properties

This keeps the existing renderer API unchanged while allowing custom renderers to consume algorithm-specific aggregate data without recomputing it from every marker.

Example:

const renderer = {
  render: (cluster, stats, map) => {
    const properties = cluster.properties;

    // Properties produced by SuperCluster map/reduce are now available here.
    console.log(properties);

    return new google.maps.Marker({
      position: cluster.position,
      label: String(cluster.count),
    });
  },
};