Four test assertions carried over from the C# suite are still commented out

Four assertions carried over from the original C# test suite are still commented out:

- `tests/test_dfsu2D.py:404` — `# TODO: Assert.AreEqual(eumUnit.eumUmeter, axis.AxisUnit);`
- `tests/test_dfsu2D.py:469` — same
- `tests/test_dfsu2D.py:806` — `# TODO: Implement?`
- `tests/test_dfs_static_item.py:113` — `# TODO: Assert.AreEqual(eumUnit.eumUmeter, axis.AxisUnit`

The three `AxisUnit` ones are a single, small check in Python:

```python
assert axis.AxisUnit == eumUnit.eumUmeter
```

If that holds, the tests get slightly stronger for free. If it does not hold, that is more interesting than the test, and worth a separate issue.

`test_dfsu2D.py:806` is vaguer and may just be a note that the C# original tested something the Python port does not cover; it should either become a real test or be removed.

Small, self-contained, and a reasonable first contribution.

Start at tests/test_dfsu2D.py lines 404, 469, and 806, plus tests/test_dfs_static_item.py line 113. Add the three specified AxisUnit assertions, then determine whether the remaining TODO should become a real test or be removed. Run the affected test files and confirm the assertions pass and no commented TODO remains.

Written by the indexing model from the issue text.