Implement differential formats as style masks

Currently differential formats are applied on load and save directly to the base style of the container. Because the context/container can be different (think tables, cells, etc), this causes duplicate differential formats to be saved. It also creates other problems.

Suggestion is to implement differential formats internally as style masks too.

Start by tracing where differential formats are applied during load and save, especially for tables and cells, and inspect the existing style representation. Define how style masks should preserve context without mutating the base style, then verify that repeated differential formats are no longer duplicated in saved workbooks.

Written by the indexing model from the issue text.