argSortRadix

private fn

size_t[] argSortRadix(Kind K)(ArrayData[]  chunks,  bool  descending,
       bool  nullsLast)

The stable radix path: collects the valid slots' radix keys and original indices, then LSD-sorts by 8-bit digits, skipping the passes whose digit is constant. Stability keeps equal keys in original order (the index tie-break of D17); descending inverts the keys so one ascending radix serves both directions.

Parameters

`K`	the (fixed-width) kind of the key column
`chunks`	the key column's chunks
`descending`	largest keys first
`nullsLast`	nulls after the valid entries

Returns

the sorted permutation of 0 .. length.