Use Kahan summation for float aggregations to reduce errors

### Description

With facet associations, we have the option of summing floats into an accumulator. This type of operation is prone to errors, which can be prevented by using summation algorithms specifically designed for floats, such as Kahan summation.

Start by locating the facet-association code that sums floats into an accumulator. Read the existing aggregation path and determine where Kahan summation should replace ordinary float addition. Done means float aggregations use the specified summation approach and the associated numerical error is reduced.

Written by the indexing model from the issue text.