lookup method
The cached frame bytes for key, or null on a miss.
Throws an ArgumentError when key is not 16 lower-case hex characters
(the shape frameKey produces).
Implementation
Future<Uint8List?> lookup(String key) async {
_checkKey(key);
final file = File('${root.path}/$key');
if (!file.existsSync()) return null;
return file.readAsBytes();
}