docs(tvix/TODO): extend NAR rendering prefetching idea

With the seekable NAR renderer, figuring out the next few blobs to
render became possible.

Change-Id: I1214302f88e6f9aba74227f84df0f964d587baf2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12652
Tested-by: BuildkiteCI
Reviewed-by: edef <edef@edef.eu>
Autosubmit: flokli <flokli@flokli.de>

tvix/docs/src/TODO.md

@@ -75,8 +75,13 @@ 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`.
+   Our seekable NAR AsyncRead implementation already removes most complexity in
+   rendering everything between blobs.
+   It should be possible to extend / write a wrapped version of it that
+   prefetches a configurable sliding window of blobs.
+   Per-blob prefetching itself is somewhat blocked until the {Chunk/Blob}Service
+   split is done, as then prefetching there 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