feat: floodlit roadside billboard carries the selected-builds case studies

src/journey/billboard-feed.ts

@@ -0,0 +1,47 @@
+/**
+ * DRIVE-IN, the part the DOM needs (initial chunk): the case-study stops and
+ * the slot the scene reads. The beats themselves (t, floods, screen kind)
+ * live in billboard-progress.ts, which only the lazy scene chunk loads.
+ */
+export interface BillboardShot {
+  /** Manifest asset id (html[data-billboard-focus] names it). */
+  id: string;
+  url: string;
+  kind: string;
+}
+
+export interface BillboardProject {
+  id: string;
+  title: string;
+  /** The route plate, e.g. "05.2 / PUBLIC / ACTIVE" (src/lib/route-plate.ts). */
+  plate: string;
+  shots: readonly BillboardShot[];
+}
+
+/**
+ * Chapter-local progress at which each case-study header crosses the
+ * activation line, from document tops (the chapter heading, each header,
+ * the next chapter's heading). The first case study takes the board as the
+ * chapter opens (stop 0), so the board is lit in the band above the chapter
+ * heading. Scroll-invariant: measure once per layout.
+ */
+export function billboardStops(chapterTop: number, nextTop: number, headerTops: readonly number[]): number[] {
+  const span = nextTop - chapterTop;
+  return span > 0 ? headerTops.map((top, i) => (i === 0 ? 0 : Math.min(1, Math.max(0, (top - chapterTop) / span)))) : [];
+}
+
+/** Index of the case study holding the board at chapter progress p: the last stop passed, else -1. */
+export const activeStop = (p: number | null, stops: readonly number[], count: number): number =>
+  p === null ? -1 : Math.min(count, stops.filter((stop) => p >= stop).length) - 1;
+
+/**
+ * The DOM publishes the projects and their stops here once per layout; the
+ * scene reads them inside its own frame. Nothing here schedules work.
+ */
+let feed: { projects: readonly BillboardProject[]; stops: readonly number[] } = { projects: [], stops: [] };
+
+export function publishBillboard(projects: readonly BillboardProject[], stops: readonly number[]): void {
+  feed = { projects, stops };
+}
+
+export const readBillboard = (): typeof feed => feed;