`order by <sorted column> DESC limit N` queries are not optimised at segment level

A query like

```sql
SELECT *
FROM table
WHERE filter
ORDER BY sorted_col DESC
LIMIT 10;
```

is not optimised and hits the generic `SelectionOrderByOperator.computePartiallyOrdered()` which inserts up to 10k elements into a priority queue to discard all but the last 10. For ascending order, the query below is roughly 3-4x faster per segment.

```sql
SELECT *
FROM table
WHERE filter
ORDER BY sorted_col ASC
LIMIT 10;
```

Start with SelectionOrderByOperator.computePartiallyOrdered() and reproduce the SQL examples for descending and ascending order. Compare their per-segment behavior and establish that the descending query avoids the generic priority-queue path while preserving the requested LIMIT 10 results.

Written by the indexing model from the issue text.