New workload: so (StackOverflow flat posts)

Port the OSB `so` workload. CC-SA-3.0 Stack Exchange flat question/answer posts. Broad full-text search and faceting on a large, familiar corpus. Lower priority than `nested` (same source data) but useful as a standalone flat-document workload.

## Tasks
- Convert OSB workload using `solr-orbit convert-workload`
- Define operations: full-text search on title/body, tag faceting, date sort
- 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/so
- Creating workloads: https://github.com/apache/solr-orbit/blob/main/CREATE_WORKLOAD_GUIDE.md

Start with CREATE_WORKLOAD_GUIDE.md and the OSB so workload, then run solr-orbit convert-workload to establish the workload structure. Review common_operations/ for reusable operations and resolve the ASF hosting dependency in #3 before finalizing corpus files. Done means the workload defines title/body search, tag faceting, date sorting, and a 1k test-mode sample corpus.

Written by the indexing model from the issue text.