BSSN-RG: A 1D Numerical Relativity Toy Model

BSSN‑RG is a lightweight, fully reproducible 1D numerical relativity toy model that implements a simplified BSSN/Z4c‑style evolution system coupled to a phenomenological scalar field. The software is designed for exploratory studies of hyperbolic PDE evolution, constraint monitoring, and numerical stability in a controlled setting. It includes a deterministic build system, automated tests, a quick‑demo mode for reviewers, and Python tools for visualising key diagnostics, providing a minimal and transparent platform for reproducible numerical experimentation.