feat(engine): semantic redaction (meaning-based sensitivity)

[martsokha]

DEVELOPER.md Section 5.3: support redaction of semantic categories: references to rare diseases, specific legal proceedings, or proprietary methodologies, where sensitivity comes from meaning in context rather than a specific identifier.

• Research semantic sensitivity detection approaches
• Implement semantic category matching
• Integrate with redaction pipeline
• Add integration test

Reference: docs/DEVELOPER.md Section 5.3