Use util.unit.mm_to_m in add_door_representation

Drops the module-local ``mm()`` helper in favour of the centralised
``ifcopenshell.util.unit.mm_to_m`` (added earlier in this PR). The
``as mm`` import alias preserves the existing call sites' readability.

Generated with the assistance of an AI coding tool.

src/ifcopenshell-python/ifcopenshell/api/geometry/add_door_representation.py

@@ -28,6 +28,7 @@ import ifcopenshell.api.geometry
 import ifcopenshell.util.unit
 from ifcopenshell.api.geometry.add_window_representation import create_ifc_window
 from ifcopenshell.util.shape_builder import ShapeBuilder, V
+from ifcopenshell.util.unit import mm_to_m as mm
 
 DOOR_TYPE = Literal[
     "SINGLE_SWING_LEFT",
@@ -43,11 +44,6 @@ DOOR_TYPE = Literal[
 SUPPORTED_DOOR_TYPES = get_args(DOOR_TYPE)
 
 
-def mm(x: float) -> float:
-    """mm to meters shortcut for readability"""
-    return x / 1000
-
-
 def create_ifc_door_lining(
     builder: ShapeBuilder, size: np.ndarray, thickness: Union[list[float], float], position: Optional[np.ndarray] = None
 ) -> ifcopenshell.entity_instance: