I think that the Ngspice netlister has some errors for DC analysis.
The param value is created but not used in (because of missing brackets).

I was able to get the simulation to run by updating the line to:

            line = (
                f".dc {param} {sweep.start} {sweep.stop} {sweep.step}\n\n"
            )

Because I believe Ngspice does not use the start,step, and stop keywords.

I also had issues afterwords with the reading of the results:

  File "/ic_tools/miniconda3/envs/Grimoire/lib/python3.10/site-packages/vlsirtools/spice/ngspice.py", line 119, in parse_results
    raise RuntimeError(msg)
RuntimeError: Cannot read results for analysis dc {
  analysis_name: "op"
  indep_name: "v.xtop.vvdc"
  sweep {
    linear {
      stop: 1
      step: 2
    }
  }
}

It cannot find the following dc enum value in the data:

I plan to finish debugging the data output.

Here is my DC testcase that I based on the existing mos test:
https://github.com/Jetsama/VLSI_Grimoire/blob/48ff1236e710a27c04ce9a3bf745fdc536ebaeae/MOS/hdl21_test.py