[WM-IMP-10] AI-generated vs. human-authored content not distinguished in ATO exports

## WM-IMP-10 — No AI vs. Human Content Marking in ATO Exports

**Category:** Improvement | **Priority:** Medium

### Observation
ATO document exports do not distinguish between AI-generated content and human-authored content. Reviewers and AOs cannot determine which sections require human validation.

### Recommendation
Mark all AI-generated content in exports with a clear indicator (e.g., watermark, metadata flag, or inline label) and require human review sign-off before export finalization.

### Acceptance Criteria
- [ ] All AI-generated prose sections marked in export (e.g., "[AI-Assisted — Reviewer Required]")
- [ ] Human-authored and human-reviewed sections unmarked or marked as validated
- [ ] Export cannot be finalized without human review sign-off on AI-generated sections
- [ ] OSCAL metadata includes AI content provenance fields where applicable