docs(m4): post-review polish — helper text + test counts

Spec-reviewer PASS pointed two factual nits:
- MitrePage helper text still referenced the old 3-column drill-down ("Pick
  a tactic on the left, then a technique..."). Reworded for the flat matrix
  with the ▸ glyph + hover-for-id idiom.
- testing-m4.md + CHANGELOG were stale at 51/12; the actual counts are 53/14
  after the GET /mitre/matrix tests landed. Reconciled.

No code-path change, no e2e fallout — DoD remains 53 pytest + 34 Playwright.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/pages/MitrePage.tsx

@@ -118,7 +118,7 @@ export function MitrePage() {
 
       {selected.length === 0 && (
         <p className="mt-3 font-mono text-2xs text-text-dim">
-          Pick a tactic on the left, then a technique, then optionally a sub-technique. Selections accumulate.
+          Click any cell to toggle. Use <span className="text-purple">▸</span> to reveal sub-techniques inline. Hover a cell for its <code className="text-purple">external_id</code>. Selections accumulate.
         </p>
       )}