Go refactor

The active runtime is Go-only. The module follows the standard root layout with cmd/, internal/, go.mod, and go.sum; Python source, dependencies, and CI were removed. The archived reference implementation remains in the legacy/python branch and python-v0.2.0 tag.

The HTTP endpoints, environment variables, output names, archive layout, and routing-file formats remain compatible apart from the required bark: route prefix. The Go renderer uses a fixed canvas, so its generated PNGs are not guaranteed to be pixel-identical to the former Matplotlib output.