- Add _format_aliases map in RDFExporter to accept 'ttl', 'nt', 'xml', 'rdf', 'json-ld' as shorthands for canonical format names
- Resolve alias at the start of export_to_rdf() before validation, leaving all existing callers unaffected
- Add TTL alias demo cell to cookbook/introduction/15_Export.ipynb
- Add tests/export/test_rdf_exporter.py covering alias parity, canonical formats, unsupported format error, and file export with format="ttl"
Closes#355
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>