feat(v4.3): offset_section_view — stepped section with multiple parallel planes

- offset_section_view(body, normal, offsets): combines section profiles at each offset
- Step lines connect boundaries between adjacent planes
- 4 new tests

include/vde/brep/brep_drawing.h

@@ -48,6 +48,16 @@ struct ProjectionView {
 [[nodiscard]] ProjectionView section_view(const BrepModel& body,
     const core::Point3D& point, const core::Vector3D& normal);
 
+/// Offset (stepped) section view — multiple parallel cutting planes
+/// Each offset defines a cutting plane at a different position along the normal
+/// direction. Results are merged with step lines at plane boundaries.
+/// @param body Body to slice
+/// @param normal Section direction (all planes share this normal)
+/// @param offsets Vector of plane positions (distance along normal from origin)
+/// @return Combined section view
+[[nodiscard]] ProjectionView offset_section_view(const BrepModel& body,
+    const core::Vector3D& normal, const std::vector<double>& offsets);
+
 /// Export views to DXF format (R12 compatible)
 /// @param filepath Output file path (.dxf)
 /// @param views Vector of views to export