docs: comprehensive README refinement - exec summary, data ingestion, SR 11-7, interoperability

- Enhanced exec summary with privacy-preserving governance value prop
- Added complete Privacy-Preserving Data Ingestion section to Layer 3
- Added SR 11-7 'Effective Challenge' enablement for vendor model validation
- Expanded use cases to include data ingestion governance
- Added Interoperability section for LangGraph and KAgentI frameworks
- Expanded all acronyms on first occurrence (BYOD, PoC, RAG, FIDO, SIEM, GRC, OCSF, SOC, SVID)
- Added horizontal rules for visual hierarchy
- Softened MRM/SR 11-7 claims to 'supports reproducibility principles'

Author: ramkri123