Download the whole app build behind a loading page

In production, a page request without the running build's cookie gets a
small loading page instead of the app. It downloads every file in
.bunext/public (listed at /build-info.json) into that build's cache,
checks they are all there, hands over to a new service worker waiting
behind an older one, sets the cookie and reloads. The service worker
then serves the app's code from the cache, so later visits load it from
the device.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/data/pwa-data.ts

@@ -9,6 +9,21 @@ export const PwaScope = "/";
 export const PwaManifestPath = "/manifest.webmanifest";
 export const PwaServiceWorkerPath = "/sw.js";
 export const PwaOfflinePath = "/offline.html";
+/**
+ * The page a browser gets instead of the app while it downloads a build
+ * it hasn't yet (`handlePrecacheGate`), generated like the offline page.
+ */
+export const PwaLoadingPath = "/loading.html";
+
+/**
+ * The running build's id and files (`handle-build-info.ts`). Each build's
+ * `.bunext/public` files are downloaded into their own cache,
+ * `<prefix><build_id>`, which holds the marker once every file is in.
+ * `public/sw.js` repeats all three.
+ */
+export const PwaBuildInfoPath = "/build-info.json";
+export const PwaPrecacheCachePrefix = "cobble-assets-";
+export const PwaPrecacheMarkerPath = "/__cobble-precached";
 
 export type PwaIcon = {
     src: string;