New workload: nested (StackOverflow questions + answers)

Port the OSB `nested` workload. CC-SA-3.0 Stack Exchange data dump with questions and nested answer objects — exercises Solr's native nested/child-document support, which has no current workload coverage.

> ⚠️ **Note:** solr-orbit may not yet have full support for parent-child document benchmarking. Check apache/solr-orbit#12 before starting implementation and consider whether this issue should be blocked on or coordinated with that one.

## Tasks
- Convert OSB workload using `solr-orbit convert-workload`
- Map nested answers to Solr child documents (`_childDocuments_`)
- Define operations: parent query with child filter, full-text on title/body
- 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/nested
- solr-orbit parent-child tracking issue: https://github.com/apache/solr-orbit/issues/12
- Creating workloads: https://github.com/apache/solr-orbit/blob/main/CREATE_WORKLOAD_GUIDE.md