[WM-IMP-13] No OSCAL pre-export validation — invalid OSCAL can be exported without warning

## WM-IMP-13 — No OSCAL Pre-Export Validation

**Category:** Improvement | **Priority:** Low

### Observation
OSCAL artifacts are exported without pre-export schema validation. Invalid OSCAL documents are silently exported and fail when consumed by downstream tools. (Related to BUG-10 which addresses the version drift root cause; this improvement addresses the validation gate.)

### Recommendation
Implement OSCAL schema validation as a required pre-export step.

### Acceptance Criteria
- [ ] OSCAL schema validation runs before every export
- [ ] Export blocked if validation fails, with specific validation errors surfaced to the user
- [ ] Validation uses official OSCAL JSON/XML schemas pinned to declared version
- [ ] Validation result logged with export audit record