New workload: geopoint (OpenStreetMap geo points)

Port the OSB `geopoint` workload. PlanetOSM geographic point coordinates (Open Database Licence). Isolates geo-point distance/bounding-box query performance more cleanly than `geonames`, which mixes full-text and spatial.

## Tasks
- Convert OSB workload using `solr-orbit convert-workload`
- Map location field to Solr `location` (LatLonPointSpatialField)
- Define operations: bbox filter, distance sort, distance faceting
- Add 1k sample corpus for test-mode
- Check whether any operations belong in `common_operations/` rather than this workload

**Depends on:** #3 (ASF dataset hosting must be resolved before corpus files can be finalised)

## References
- OSB workload: https://github.com/opensearch-project/opensearch-benchmark-workloads/tree/main/geopoint
- Creating workloads: https://github.com/apache/solr-orbit/blob/main/CREATE_WORKLOAD_GUIDE.md