docs(tvix/TODO): document blob prefetching dependency

This depends on the ChunkReader work.

Change-Id: I38878d0f822c312151131e55baee4db6ef1c3650
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12142
Tested-by: BuildkiteCI
Reviewed-by: Ilan Joselevich <personal@ilanjoselevich.com>
Autosubmit: flokli <flokli@flokli.de>

tvix/docs/src/TODO.md

@@ -89,6 +89,8 @@ correctness:
    This is somewhat the "spiritual counterpart" to our sequential ingestion
    code (`ConcurrentBlobUploader`, used by `ingest_nar`), which keeps
    "some amount of outgoing bytes" in memory.
+   This is somewhat blocked until the {Chunk/Blob}Service split is done, as then
+   prefetching would only be a matter of adding it into the one `BlobReader`.
 
 ### Error cleanup
  - Currently, all services use tvix_castore::Error, which only has two kinds