Investigate how to examine regression data fixtures outside of pytest

[jvshields]

It would be nice to be able to look at the data the in our fixtures in a notebook for more flexible testing purposes

[SS-9098]

From what I understand, the regression data is in the tardisbase repository in tardisbase/testing/regression_data.
It seems that class is intentionally pytest-bound (request object, CLI flags, write-status hooks), so reusing it outside pytest would be brittle.

A cleaner approach might be to add a small, read-only API alongside it that mirrors the on-disk layout but has no pytest dependency. This would allow regression data to be inspected in notebooks or scripts using the same regression root path passed via --tardis-regression-data.

For example, a RegressionDataReader living next to RegressionData could provide helpers like load_dataframe, load_ndarray, etc., while leaving the existing pytest workflow completely unchanged.

I’m happy to open a PR adding such a reader if this direction sounds reasonable.

[arybhatt4533]

I'm interested in helping with the RegressionDataReader implementation, especially the load_dataframe and load_ndarray helpers. Should we start by drafting the API structure?
@SS-9098