enhancement(consistency-validator): flag non-monotonic changelog modified-list frontmatter

**Severity:** LOW
**Agent owner:** consistency-validator

## Evidence

switchboard-blue Wave-5 wave-adversarial closed at CONVERGED (3 clean passes across 6-pass trajectory). Post-convergence hygiene sweep discovered THREE spec artifacts with scrambled `modified:` frontmatter version lists that survived every adversary/consistency-validator pass:

- ARCH-11-verification-coverage-matrix.md: v1.6→v1.11→v1.10→v1.9→v1.8→v1.7→v1.12 (not monotonic in either direction)
- ARCH-04-admission-security.md: v1.13 inserted before v1.9 and v1.10
- error-taxonomy.md: v3.x block reversed (v3.9→v3.1) while v2.x block was v2.1→v2.9 ascending

All three are trivially mechanical to check (parse frontmatter YAML `modified:` array, verify monotonic order).

The invariant went unchecked across all consistency-validator passes, adversary passes L1/L2/L3, and spec-reviewer passes. Currently a documentation-quality bug, but would become a real problem if any tooling uses `modified:` order to compute "current active version" or similar.

## Recommendation

Add to consistency-validator rubric — for every spec artifact with a `modified:` array, verify entries are in monotonically decreasing version order (newest first) OR consistently ascending, matching the project convention. Flag any list where adjacent entries are not comparable or where ordering flips direction.