feat(feature): GLCM texture features + regionprops extra_properties - #49

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="cubic/feature/texture.py" line_range="246-254" />
<code_context>
+        for unit in _unit_offsets(image.ndim)
+    ]
+
+    totals = {prop: 0.0 for prop in _PROPS}
+    for off in offsets:
+        glcm = _direction_matrix(quant, off, levels, mask, symmetric, normed)
+        props = _haralick_props(glcm, levels)
+        for prop in _PROPS:
+            totals[prop] += props[prop]
+
+    n = len(offsets)
+    return {prop: totals[prop] / n for prop in _PROPS}
</code_context>
<issue_to_address>
**issue:** Averaging without handling empty offsets assumes at least one valid direction

This relies on `offsets` being non-empty (`n > 0`). If upstream validation or `_unit_offsets` change, this could become a divide-by-zero. Consider an explicit guard (e.g., raise `ValueError` when `n == 0`) to fail fast and keep behavior well-defined.
</issue_to_address>

### Comment 2
<location path="tests/feature/test_voxel.py" line_range="57-66" />
<code_context>
+    assert keys.index("centroid-0") < keys.index("label")
+
+
+def test_regionprops_table_extra_properties() -> None:
+    """extra_properties callables are forwarded and keyed by function name."""
+
+    def intensity_range(regionmask: np.ndarray, intensity: np.ndarray) -> float:
+        values = intensity[regionmask]
+        return float(values.max() - values.min())
+
+    labels = np.array([[0, 1, 1], [2, 2, 0]], dtype=np.int32)
+    intensity = np.array([[0, 10, 20], [5, 5, 0]], dtype=np.float32)
+
+    out = voxel.regionprops_table(
+        labels,
+        intensity_image=intensity,
+        properties=["area"],
+        extra_properties=(intensity_range,),
+    )
+
+    assert "intensity_range" in out
+    by_label = dict(zip(out["label"].tolist(), out["intensity_range"].tolist()))
+    assert by_label[1] == 10.0  # intensities {10, 20}
+    assert by_label[2] == 0.0  # intensities {5, 5}
</code_context>
<issue_to_address>
**suggestion (testing):** Add a companion test for `regionprops` forwarding `extra_properties`, not just `regionprops_table`.

This only exercises forwarding via `regionprops_table`. Please also add a test that calls `voxel.regionprops(..., extra_properties=(func,))` and checks that the returned regions expose the custom attribute and that its values per label are correct, so both APIs stay covered and in sync.

Suggested implementation:

```python
def test_regionprops_table_extra_properties() -> None:
    """extra_properties callables are forwarded and keyed by function name."""

    def intensity_range(regionmask: np.ndarray, intensity: np.ndarray) -> float:
        values = intensity[regionmask]
        return float(values.max() - values.min())

    labels = np.array([[0, 1, 1], [2, 2, 0]], dtype=np.int32)
    intensity = np.array([[0, 10, 20], [5, 5, 0]], dtype=np.float32)

    out = voxel.regionprops_table(
        labels,
        intensity_image=intensity,
        properties=["area"],
        extra_properties=(intensity_range,),
    )

    assert "intensity_range" in out
    by_label = dict(zip(out["label"].tolist(), out["intensity_range"].tolist()))
    assert by_label[1] == 10.0  # intensities {10, 20}
    assert by_label[2] == 0.0  # intensities {5, 5}


def test_regionprops_extra_properties() -> None:
    """extra_properties callables are forwarded and exposed as attributes."""

    def intensity_range(regionmask: np.ndarray, intensity: np.ndarray) -> float:
        values = intensity[regionmask]
        return float(values.max() - values.min())

    labels = np.array([[0, 1, 1], [2, 2, 0]], dtype=np.int32)
    intensity = np.array([[0, 10, 20], [5, 5, 0]], dtype=np.float32)

    regions = voxel.regionprops(
        labels,
        intensity_image=intensity,
        extra_properties=(intensity_range,),
    )

    # Ensure custom property is exposed as an attribute and values are correct per label.
    by_label = {region.label: getattr(region, "intensity_range") for region in regions}
    assert by_label[1] == 10.0  # intensities {10, 20}
    assert by_label[2] == 0.0  # intensities {5, 5}

```

This patch assumes that `np` (NumPy) and `voxel` are already imported in `tests/feature/test_voxel.py`, consistent with the existing tests. If they are not, corresponding imports should be present at the top of the file (e.g., `import numpy as np` and the appropriate import for `voxel`).
</issue_to_address>