feat: scroll-driven maintenance window set-piece in the infrastructure chapter

src/components/AppShell.vue

@@ -18,6 +18,7 @@ import { useAudioIntent } from '@/composables/useAudioIntent';
 import { useCardEntrances } from '@/composables/useCardEntrances';
 import { useJourneySections } from '@/composables/useJourneySections';
 import { useKeyboardNavigation } from '@/composables/useKeyboardNavigation';
+import { useMaintenanceWindow } from '@/composables/useMaintenanceWindow';
 import { useMotionPreferences } from '@/composables/useMotionPreferences';
 import { useRouteMemory } from '@/composables/useRouteMemory';
 import { assetUrl } from '@/content/asset-url';
@@ -81,6 +82,8 @@ const journeyBinding = useJourneySections(
   computed(() => motion.policy.value.allowSceneRequest),
 );
 const journey = journeyBinding.snapshot;
+/* The 03:12 window (Infrastructure): the plate's sub-line and the diagram steps. */
+const maintenanceSubline = useMaintenanceWindow(journey);
 useCardEntrances();
 const route = useRouteMemory(canonicalSectionIds, () => journey.value.activeSectionId);
 const resume = computed(() => {
@@ -258,6 +261,7 @@ onUnmounted(() => removePageListeners());
     :exit="exitFor(journey.activeSectionId)"
     :next-label="nextLabelFor(journey.activeSectionId)"
     :next-exit="nextExitFor(journey.activeSectionId)"
+    :subline="maintenanceSubline"
   />
   <header class="site-header">
     <SiteNavigation