Add RxNorm drug normalization via UMLS

[AlexMikhalev]

Summary

Use RxNorm vocabulary from UMLS for brand-to-generic drug mapping, dosage form normalization, and ingredient extraction.

Details

• Clinical text often uses brand names (Keytruda) vs generic (Pembrolizumab) interchangeably
• RxNorm provides canonical drug concept hierarchy: ingredient -> clinical drug -> branded drug
• Improves entity extraction accuracy for varied drug name mentions
• Enables dosage form awareness (tablet vs injection vs topical)

Acceptance Criteria

• Parse RXNCONSO and RXNREL from UMLS RxNorm subset
• Build brand-to-generic mapping table
• Integrate normalization into entity extraction pipeline
• Test with drug name variants in evaluation cases

Priority: P3