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

tests/brep/test_brep_drawing.cpp

@@ -242,3 +242,32 @@ TEST(BrepDrawingTest, ProjectionView_TotalSegments) {
     v.hidden_lines = {{0,1,0,0, true}};
     EXPECT_EQ(v.total_segments(), 3u);
 }
+
+// ═══════════════════════════════════════════════════════════
+//  Offset (stepped) section views
+// ═══════════════════════════════════════════════════════════
+
+TEST(OffsetSectionTest, SingleOffset) {
+    auto box = make_box(4, 4, 4);
+    auto view = offset_section_view(box, Vector3D(0, 0, 1), {0.0});
+    EXPECT_FALSE(view.segments.empty());
+    EXPECT_EQ(view.name, "offset_section");
+}
+
+TEST(OffsetSectionTest, MultipleOffsets) {
+    auto box = make_box(4, 4, 4);
+    auto view = offset_section_view(box, Vector3D(0, 0, 1), {-1.0, 0.0, 1.0});
+    EXPECT_GE(view.segments.size(), 3u);  // section + step lines
+}
+
+TEST(OffsetSectionTest, EmptyOffsets) {
+    auto box = make_box(4, 4, 4);
+    auto view = offset_section_view(box, Vector3D(0, 0, 1), {});
+    EXPECT_TRUE(view.segments.empty());
+}
+
+TEST(OffsetSectionTest, CylinderSection) {
+    auto cyl = make_cylinder(2, 6, 32);
+    auto view = offset_section_view(cyl, Vector3D(0, 0, 1), {-2.0, 0.0, 2.0});
+    EXPECT_GE(view.total_segments(), 3u);
+}