refactor(tvix/store/composition): rename 'default' to 'root'

This becomes the root of the composition. `default` implies we can
directly access anything else, which we cannot. `root` makes this more
understandable, and it's all internal only anyways.

Change-Id: I297511bc05a7c32c59510b9d192b40d1bd937b5f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12746
Reviewed-by: yuka <yuka@yuka.dev>
Tested-by: BuildkiteCI

tvix/castore/src/blobservice/tests/utils.rs

@@ -30,7 +30,7 @@ pub async fn make_grpc_blob_service_client() -> Box<dyn BlobService> {
     let mut maybe_right = Some(right);
 
     Box::new(GRPCBlobService::from_client(
-        "default".into(),
+        "root".into(),
         BlobServiceClient::new(
             Endpoint::try_from("http://[::]:50051")
                 .unwrap()