Document thread-safety of Arrays.parallelSort in sortByColumns

[Copilot]

Addresses review question about thread-safety of Arrays.parallelSort usage in the single-column sort optimization path.

Changes

• Added inline comment documenting why parallelSort is thread-safe:
  • Comparator performs concurrent reads only via column[index]
  • Column access is side-effect-free (values[index] delegation)
  • Standard List implementations support concurrent reads without modification

The parallelSort implementation is correct as-is. The locally-created permutation array and read-only column access satisfy thread-safety requirements.

val permutation = Array(column.size()) { it }
// parallelSort is thread-safe here: the comparator only performs concurrent reads from the column,
// and column.get() is a simple, side-effect-free access to an underlying List.
// Standard List implementations (ArrayList, etc.) are safe for concurrent reads
// as long as there are no concurrent modifications to the dataframe.
Arrays.parallelSort(permutation, finalComparator)

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