rankOf

Series rankOf(Series series,  RankMethod method =  RankMethod.AVERAGE,
       bool  descending =  false)

Ranks the valid values under the total order (ADR-2026-10-23-11): 1-based; ties receive the method's position; null slots stay null; descending reverses the value order, not the tie semantics (FIRST still means original row order). One stable permutation, one ordered equal-run walk, one scatter back - the identical result at every thread count (the sort's determinism guarantees it; rank never depends on quantile machinery).

Parameters

`series`	the column to rank
`method`	the tie method (`AVERAGE` yields FLOAT64, the rest INT64)
`descending`	largest values first

Returns

the rank Series (FLOAT64 for AVERAGE, INT64 otherwise).