Admit point-group MgO EOS runs safely

[benjipeng]

Summary

• add a full-versus-reduced energy oracle before point-group MgO EOS admission
• fall back to the full k-point mesh when method validation fails
• record the rejected CP2K 2026.1 UZH transferability diagnostic without weakening scientific gates

Verification

• uv run ruff check src/mlx_atomistic/benchmarks/dft_mgo_eos_runner.py tests/test_dft_mgo_eos.py
• uv run pytest -q tests/test_dft_mgo_eos.py
• uv run python scripts/sync_site_docs.py --check
• uv run --group docs python scripts/gen_api_docs.py
• uv run python -m mlx_atomistic.benchmarks.dft_gth_transferability --json