feat: overhead gantry top bar with a docking sign window

The top bar becomes a steel gantry. Its centre window is a mask cut-out the
3D exit sign docks into; between signs the window shows the dash read-out.
The drive controls move into the right bay, and the reading viewport is
clipped at the beam, so the empty band under the bar is gone.

src/components/AppShell.vue

@@ -253,6 +253,8 @@ onUnmounted(() => removePageListeners());
     :journey="journey"
     :section-label="activeSectionLabel"
     :exit="exitFor(journey.activeSectionId)"
+    :next-label="nextLabelFor(journey.activeSectionId)"
+    :next-exit="nextExitFor(journey.activeSectionId)"
   />
   <header class="site-header">
     <SiteNavigation