New workload: PMC (PubMed Central scientific articles)

Port the OSB `pmc` workload. CC-BY-2.0 dataset of scientific articles — adds proper full-text relevance benchmarking (BM25, phrase search, high-cardinality text fields) which is the biggest gap in the current workload set.

## Tasks
- Convert OSB workload using `solr-orbit convert-workload`
- Adapt schema to Solr field types (text_general, string, date)
- Define operations: default search, phrase search, term faceting
- Add 1k sample corpus for test-mode
- Check whether any operations belong in `common_operations/` rather than this workload
- Document dataset licence in workload README

**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/pmc
- Creating workloads: https://github.com/apache/solr-orbit/blob/main/CREATE_WORKLOAD_GUIDE.md