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/SiteNavigation.vue

@@ -93,6 +93,22 @@ onBeforeUnmount(() => {
   compactQuery = null;
 });
 
+/** The gantry's sign window drives on to the next exit (or back to the
+ * start from the destination), exactly like following that exit's link. */
+const nextIndex = computed(() =>
+  activeIndex.value >= props.items.length - 1 ? 0 : activeIndex.value + 1,
+);
+const windowLabel = computed(() => {
+  const next = props.items[nextIndex.value];
+  if (next === undefined) return "Drive on";
+  return nextIndex.value === 0
+    ? `Back to the start: ${next.label}`
+    : `Drive to the next exit: ${next.label}`;
+});
+const driveOn = (): void => {
+  list.value?.querySelectorAll<HTMLAnchorElement>("a")[nextIndex.value]?.click();
+};
+
 const navigate = (event: MouseEvent): void => {
   props.onSectionLinkClick(event, true);
   if (window.matchMedia("(max-width: 767px)").matches)
@@ -117,5 +133,13 @@ const navigate = (event: MouseEvent): void => {
         </li>
       </ul>
     </details>
+    <button
+      v-if="enhanced"
+      type="button"
+      class="gantry-window"
+      :aria-label="windowLabel"
+      :title="windowLabel"
+      @click="driveOn"
+    ></button>
   </nav>
 </template>