Hotfix HF dataset parity tests and build artifacts

[Am1n3e]

Summary

• fix HF dataset parity path by preserving sites as string lists in build artifacts
• remove intermediate full.json and hard.json from build output to avoid local loader schema conflicts
• require explicit --hf-dataset-ref in HF parity tests via pytest.UsageError
• include supporting HF dataset docs/spec files added in this branch

Validation

• uv run inv dev.release.build-hf-dataset --version v1.2.2
• uv run pytest tests/dataset/test_hf_dataset.py --hf-dataset-ref ./output/build/hf_dataset/