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/directoryservice/object_store.rs

@@ -64,7 +64,7 @@ impl ObjectStoreDirectoryService {
         let (object_store, path) = object_store::parse_url_opts(url, options)?;
 
         Ok(Self {
-            instance_name: "default".into(),
+            instance_name: "root".into(),
             object_store: Arc::new(object_store),
             base_path: path,
         })

tvix/castore/src/directoryservice/redb.rs

@@ -56,7 +56,7 @@ impl RedbDirectoryService {
         create_schema(&db)?;
 
         Ok(Self {
-            instance_name: "default".into(),
+            instance_name: "root".into(),
             db: Arc::new(db),
         })
     }

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

@@ -33,7 +33,7 @@ pub async fn make_grpc_directory_service_client() -> Box<dyn DirectoryService> {
     // Create a client, connecting to the right side. The URI is unused.
     let mut maybe_right = Some(right);
     Box::new(GRPCDirectoryService::from_client(
-        "default".into(),
+        "root".into(),
         DirectoryServiceClient::new(
             Endpoint::try_from("http://[::]:50051")
                 .unwrap()