Adds the `desc_equilibrium` config: for 156,357 of the 182,222 boundaries in `default`, the vacuum ideal-MHD equilibrium re-solved with DESC, initialized from the corresponding `vmecpp_wout` equilibrium. These are the equilibria used to generate the CoilStellaration coilsets.
- `desc_equilibrium/part.*.parquet`: `plasma_config_id`, `id`, `vmecpp_wout_id`, `desc_normalized_mean_absolute_force_error`, `desc_normalized_max_absolute_force_error`, `json`.
- `desc_equilibrium/id_to_file_map.parquet`: `id` -> `path`, as for `vmecpp_wout`.
- `default`: new column `misc.desc_equilibrium_id` (null where no DESC equilibrium is available); all other columns are unchanged.
- README: config, features, description and usage example.
The JSON validates into `constellaration.mhd.desc_utils.DescEquilibrium` (GitHub PR to follow).
