fix(bench): keep hero heading in renderAllEmpty for UX consistency

When the snapshot fallback chain fully fails, renderAllEmpty was emitting
just an error <p> for the hero panel while still emitting <h4> titles for
each throughput panel — an inconsistent layout where the hero heading
vanishes on error but section headings stay. Add the standard
'Throughput at a glance' <h3> before the error message so the page
structure is identical between healthy and empty states.

Addresses Momus BOT-A2 (Low).

Author: elijahr

docs/assets/bench-charts.js

@@ -1019,6 +1019,8 @@
     const heroHost = document.getElementById('bench-hero');
     if (heroHost) {
       heroHost.innerHTML = '';
+      heroHost.appendChild(el('h3', { class: 'bench-hero-heading' },
+        'Throughput at a glance'));
       heroHost.appendChild(el('p', { class: 'bench-hero-empty' },
         'Bench snapshot pending — ' + reason));
     }