Precache all thumbnails and pinned originals inside open() (closes #48)

Precaches aggressively inside open(): every thumbnail (newest first, through the shared thumbnail pool, never evicted; visible/ahead requests preempt the background fill) and the pinned originals — the favorites album plus every file within precacheOriginalsDays (default 7) of the newest takenAt — through the content pool. The pinned set integrates with the #47 eviction hook; when the window moves or a favorite is removed, files become ordinary evictable originals. open() options precacheThumbnails/precacheOriginals/precacheOriginalsDays; progress via onProgress/status().

Model: opus-4-8

test/cli/backup.test.ts

@@ -142,6 +142,10 @@ const openLibrary = (source: ContentSource): Promise<Library> =>
         cacheDirectory: join(root, "cache"),
         contentSource: source,
         refreshIntervalSeconds: 3600,
+        // These tests count exact fetches; the background precache (#48) would
+        // add its own, so it is off here (it is covered in precache.test.ts).
+        precacheThumbnails: false,
+        precacheOriginals: false,
     });
 
 const readLedger = (