style: fade the exit plate in over 160 ms under reduced motion

src/components/JourneyHud.vue

@@ -20,10 +20,13 @@ const next = computed(() =>
    a route rule that fills along its lower edge. */
 const route = computed(() => Math.min(1, Math.max(0, props.journey.routeProgress)));
 const routePercent = computed(() => `${Math.round(route.value * 100)}%`);
+/* The plate is keyed by chapter, so a new chapter is a new element: under
+   reduced motion it fades in over 160 ms (gantry.css) instead of swapping. */
 </script>
 
 <template>
   <aside
+    :key="journey.activeSectionId"
     class="journey-hud"
     aria-hidden="true"
     :data-exit="exit"