feat(engine): synthetic data replacement for redacted content

[martsokha]

DEVELOPER.md Section 5.4: support replacement of redacted content with realistic synthetic alternatives (generated names, addresses, dates) that preserve statistical and structural properties while eliminating re-identification risk.

• Design synthetic replacement strategy per entity type
• Implement synthetic data generation
• Integrate as a redaction output mode
• Add integration test

Reference: docs/DEVELOPER.md Section 5.4