Make it possible to reuse the results of a `diff`

It would be useful to be able to reuse the results of a diff, ideally within OpenCodeLists or otherwise by downloading it, including the match status, to enable further codelist development or analyses.

At present the results are in list form, separated into sections (LHS only, codes in common, etc) but with codes listed as <Description> (<code>) making the results difficult to reuse for anything else.

I'd like to be able to reuse the results to:

• run further analyses on the matched or unmatched codes, e.g. in OpenCodeCounts,
• create new codelists e.g. using only the unmatched codes from the LHS or RHS only, the codes in common, or combining all codes found in either codelist. (ideally creating new codelists from a diff could be done directly within opencodelists rather than by downloading and uploading).

The function to add a category column within OCC (#507) could be one way of implementing part of this feature - i.e. add the match status (LHS, RHS, in common) as a category column within a new combined codelist created from the diff.

Start by reviewing how OpenCodeLists currently presents diff results and the proposed category-column work in OpenCodeCounts issue #507. Define whether the result should be downloadable, reusable within OpenCodeLists, or both, and preserve match status for LHS-only, RHS-only, and common codes. Done means the selected diff results can support further analysis or creation of new codelists.