copy method
Asks the node to copy from to to under the served root, reusing the
node-side bytes — but only when from still hashes to hash.
Returns true when the node verified and copied; false when from
drifted or vanished, in which case the caller falls back to a byte
transfer. A false outcome is a normal reply, not an error.
Implementation
Future<bool> copy(String from, String to, String hash) async {
final reply = await _call(
DriveOp.copy,
fields: {'from': from, 'to': to, 'hash': hash},
);
return reply.header['copied'] == true;
}