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/library/store.ts

@@ -58,6 +58,12 @@ export class MetadataStore {
     userID = 0;
     collectionsSinceTime: Microseconds = 0;
 
+    // True when `load` populated this store from a valid existing file; false
+    // on a first run or a missing/corrupt/wrong-version file that loaded empty.
+    // `Library.open` reads it to decide whether the first refresh may run in
+    // the background (an existing copy already serves reads) or must be awaited.
+    loadedFromDisk = false;
+
     private readonly collections = new Map<number, Collection>();
     private readonly files = new Map<string, EnteFile>();
 
@@ -83,6 +89,7 @@ export class MetadataStore {
             if (parsed.schemaVersion !== METADATA_SCHEMA_VERSION) {
                 return store;
             }
+            store.loadedFromDisk = true;
             store.userID = parsed.userID ?? 0;
             store.collectionsSinceTime = parsed.collectionsSinceTime ?? 0;
             for (const stored of parsed.collections ?? []) {