fix: resolve TTL export alias failure and add RDF notebook example (#355)

- 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>

cookbook/introduction/15_Export.ipynb

@@ -178,6 +178,13 @@
     "rdf_exporter.export(kg, \"output.ttl\", format=\"turtle\")"
    ]
   },
+  {
+   "cell_type": "code",
+   "source": "# TTL alias: format=\"ttl\" is equivalent to format=\"turtle\"\nrdf_data = {\n    \"entities\": [\n        {\"id\": \"e1\", \"text\": \"Apple Inc.\", \"type\": \"ORG\", \"confidence\": 0.95},\n        {\"id\": \"e2\", \"text\": \"Steve Jobs\", \"type\": \"PERSON\", \"confidence\": 0.97},\n    ],\n    \"relationships\": [\n        {\"source_id\": \"e2\", \"target_id\": \"e1\", \"type\": \"founded_by\", \"confidence\": 0.91},\n    ],\n}\n\nrdf_exporter.export(rdf_data, \"output.ttl\", format=\"ttl\")\n\nresult = rdf_exporter.validate_rdf(rdf_data)\nprint(f\"Valid: {result['valid']}\")",
+   "metadata": {},
+   "execution_count": null,
+   "outputs": []
+  },
   {
    "cell_type": "markdown",
    "metadata": {},
@@ -363,4 +370,4 @@
  },
  "nbformat": 4,
  "nbformat_minor": 2
-}
+}