Configuration: Define order for config sources that have same ordinal

Today the order for 2 config sources that have the same ordinal is not defined. We need to define the order for such a case and implement it.

Start by tracing how configuration sources are collected and ordered, then inspect how ordinal values are compared. Determine the expected deterministic tie-breaker for equal ordinals and add coverage demonstrating that the selected order remains stable.

Written by the indexing model from the issue text.