Extending the syntax scheme

To add a new syntacatic relation to arsgrammatica:

1. Add the new label to RelationLabel in arsgrammatica/models.py.
2. Describe when to use it in SyntaxAnalysis’s docstring in arsgrammatica/latin_syntax_dspy.py, following the pattern of the existing relations (which token gets relatedtoken1/relationship1, which gets the corresponding value on the other end).
3. Add a gold example exercising it to tests/fixtures/gold_examples.py and re-run pytest to confirm the models still validate before trying it against the real LM.