Add ChemOnt classification enricher

## Summary - expose `classified_by` on chemical entities with `CHEMONTID` support - add a local `chemont` enricher that maps computed InChIKeys to the most specific ClassyFire/ChemOnt class - add `prepare-chemont` to materialize the Zenodo TSV release as indexed DuckDB for point lookups or Parquet for scan workflows - regenerate LinkML artifacts and cover DuckDB, Parquet, CLI, and registry behavior ## Verification - `make gen` - `make test-examples` - `uv run linkml-validate --schema src/chemrof/schema/chemrof.yaml --target-class SmallMolecule <temp classified_by example>` - `uv run pytest --ignore=tests/chem_model_test.py` Note: full `uv run pytest` still fails during collection because `tests/chem_model_test.py` imports missing module `chem`; this is unrelated to this change. @dragon-ai-agent