[Roadmap] Regression testing with GNU Guix

## Objective

Set up reproducible regression testing across the control-toolbox ecosystem using [GNU Guix](https://guix.gnu.org/).

## Context

Guix provides declarative, reproducible package management. Using it for regression testing would ensure that tests are reproducible across different environments and over time.

## Why

Reproducibility is critical for scientific software. Guix allows pinning exact versions of all dependencies (including system libraries), making it possible to reproduce test results months or years later. This is particularly important for a research ecosystem like control-toolbox.

## References

- [GNU Guix](https://guix.gnu.org/)
- [Roadmap comment](https://github.com/orgs/control-toolbox/discussions/68#discussioncomment-17584843)