joinValueDtypes

private fn

DataType joinValueDtypes(DataType a,  string an,  DataType b,
       string bn)

Joins two stacked value dtypes: identical dtypes stay; numeric dtypes join in the ADR-2026-10-24-19 lattice (all-integer INT64 with exact widening, any float FLOAT64, UINT64 meeting a signed integer FLOAT64 - the approximation is disclosed here); every other mix rejects, naming both columns.

Parameters

`a`	one dtype
`an`	the column `a` came from (for the error)
`b`	the other dtype
`bn`	the column `b` came from

Returns

the joined dtype.

Throws

DataFrameException when no exact-or-disclosed join exists.