Parallel radix sort for large render queues

Parallel radix sort for large render queues

The radix path now uses the paint executor: key build, the 8 LSD passes
(per-chunk histograms, digit-major/chunk-minor offsets, per-chunk
scatter) and the final permute all run chunked in parallel. Stability is
preserved exactly, so the output is bit-identical to the serial sort for
any chunk count (pinned by a new RadixLongSortTest property test over
sizes 0..250k x chunk counts 1..8) — golden-image determinism intact.

Also deletes the now-unreachable parallelMergeSort/runSortTasks/
mergeRuns (~100 LOC) and routes the no-executor (headless) path through
the serial radix instead of Arrays.parallelSort.

Measured on a 24-core desktop (450k pairs, SortBench/SortSweep harnesses):
sort phase ~7 ms serial -> ~4-5 ms parallel (1.2-1.8x; bandwidth-bound,
frequency-capped test machine). Key-build/permute parallelization comes
on top in the full pipeline.