Library.open with a transparent background refresh (closes #42)

Library.open loads metadata.json and serves reads from RAM: an empty cache awaits the first refresh, an existing cache returns at once and refreshes in the background so an unreachable server never stalls open(). A background timer refreshes every refreshIntervalSeconds (default 3), diffing only changed albums via the resumable cursor+tombstone enumerators and rewriting metadata.json only when something changed. A refresh failure is invisible to reads and surfaced via status()/onProgress; a failed save keeps status().lastError set and retries until one lands, so a stale disk is never masked. No sync()/refresh()/serverReachable surface; status() and close() included.

Model: opus-4-8

src/index.ts

@@ -33,6 +33,15 @@ export {
     requestEmailOTP,
     submitEmailOTP,
 } from "./auth/login.js";
+export {
+    Library,
+    DEFAULT_REFRESH_INTERVAL_SECONDS,
+    type LibraryClient,
+    type LibraryOptions,
+    type LibraryStatus,
+    type RefreshEvent,
+    type RefreshProgressCallback,
+} from "./library/index.js";
 export { decryptCollection, decryptFile } from "./model/index.js";
 export { downloadFile, downloadThumbnail } from "./download/index.js";
 export type {