[WM-IMP-3] No runtime catalog integrity check — OSCAL control catalog not validated at startup

## WM-IMP-3 — No Runtime Catalog Integrity Check

**Category:** Improvement | **Priority:** High

### Observation
The OSCAL control catalog is not validated at startup or runtime. Corrupted or incomplete catalog data can silently degrade control lookup, assessment, and document generation.

### Evidence
- Issue #636 — no runtime catalog integrity check (O2 finding)

### Recommendation
Implement a startup integrity check that validates the control catalog against the expected OSCAL schema and record count.

### Acceptance Criteria
- [ ] Startup check validates catalog record count against expected baseline
- [ ] Catalog validated against OSCAL schema at startup
- [ ] Application refuses to start (or enters degraded mode with alert) if catalog is invalid
- [ ] Integrity check results logged