Preserve CTD taxon metadata

[cbizon]

Summary

• Enable Babel taxon retrieval during normalization and use the top-level NodeNorm taxa field for node taxon.
• Remove CTD gene node NCBITaxon properties and emit CTD row-level taxa as edge taxon CURIEs.
• Preserve CTD row-level taxa during graph merges by adding edge_merging_attributes: [taxon] to CTD-containing graph specs and downstream subgraph merge specs.

Validation

• uv run --with beautifulsoup4 pytest tests/test_graph_spec.py tests/test_merging.py tests/test_normalization.py tests/test_kgx_file_normalizer.py tests/test_ctd_loader.py -> 70 passed.
• Rebuilt CTD KGX graph with taxon-aware merging: build status stable, QC passed, 26,491 nodes, 186,376 edges.
• Rebuilt CTD output has 10,659 node taxon properties, 83,806 edge taxon properties, zero NCBITaxon/taxonid node or edge properties, and zero invalid taxon CURIE values.
• Taxon-aware merge check found zero source merge groups with multiple distinct taxa.

Notes

• A local Neo4j-output build reached KGX build and QC but failed afterward because neo4j-admin is not installed locally; KGX validation completed successfully.

[cbizon]

Fixes #412