feat(v3.4): integrate face splitting into B-Rep boolean operations

Replace centroid-only classification (classify_point_mesh on full face fragments)
with split-and-classify approach using split_face_by_plane() from v3.3:

- Add include for brep_face_split.h
- Add face_normal() helper (extracts surface normal at parametric mid-point)
- Add split_and_classify_faces(): splits each face of body A by the planes of
  every face in body B, then classifies each resulting fragment individually
- Refactor brep_union, brep_intersection, brep_difference to use the new
  split-and-classify pipeline
- Add test: Union_SelfUnion_WithSplitting

This produces more precise boolean results by ensuring face fragments that
straddle the boundary are properly split before classification.

tests/brep/test_brep_boolean.cpp

@@ -153,6 +153,17 @@ TEST(BrepBooleanTest, Union_Commutative) {
     EXPECT_TRUE(r2.is_valid());
 }
 
+// ═══════════════════════════════════════════════════════════
+//  Boolean operations with face-plane splitting (v3.4)
+// ═══════════════════════════════════════════════════════════
+
+TEST(BrepBooleanTest, Union_SelfUnion_WithSplitting) {
+    auto box = make_box(2, 2, 2);
+    auto result = brep_union(box, box);
+    EXPECT_TRUE(result.is_valid());
+    // Self-union should produce a valid result
+}
+
 TEST(BrepBooleanTest, Intersection_Commutative) {
     auto box1 = make_box(2, 2, 2);
     auto box2 = make_box(2, 2, 2);