doc: record pushing columnar containers into TopK as a TODO

TopK renders its stages over `Vec` containers, so the columnar output edge
is produced by a single encode at the end of the plan. Note at that seam that
the encode disappears once the stages themselves carry columnar containers.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: antiguru

src/compute/src/render/top_k.rs

@@ -669,6 +669,10 @@ where
 /// therefore injective and the output carries no within-batch duplicates, so a
 /// non-consolidating `ColumnBuilder` matches the prior `map`. The row is pushed
 /// borrowed, materializing no owned `Row` per record.
+///
+/// TODO: TopK renders its stages over `Vec` containers, so this encode sits at
+/// the very end of the plan. Pushing columnar containers down through
+/// `build_topk` and the monotonic path would remove it.
 fn topk_result_to_columnar<'s, T>(
     collection: VecCollection<'s, T, (Row, Row), Diff>,
 ) -> CollectionEdge<'s, T>