The value at quantile (on a 0-1 scale, not 0-100) of the NON-NULL values.
index optionally reports which element that was; n_null how many were excluded.
Nulls are excluded from the population, not placed in it -- unlike every other
operation in this module, which is why this one takes neither descending nor
nulls_first: there is no null tier to position, and a descending quantile is just
1 - quantile.
rounding= selects how a fractional position is resolved: "nearest" (the default),
"down" or "up", matched case-insensitively. An unrecognized token aborts.
Aborts when EVERY value is null: there is no value to return, and no sentinel exists
across all ten types. n_null is for PARTIAL nullness; the all-null case never
reaches it. Guard with count(mask) (or a column's own null count) if that matters.