fix: replace Eigen::SelfAdjointEigenSolver with cross product plane fitting

src/brep/brep_boolean.cpp

@@ -521,18 +521,14 @@ std::vector<BrepModel> split_face_by_face_ssi(
     for (const auto& p : valid_curve->points) centroid += p;
     centroid /= static_cast<double>(valid_curve->points.size());
 
-    // Fit plane via SVD on intersection points
-    Eigen::Matrix3d cov = Eigen::Matrix3d::Zero();
-    for (const auto& p : valid_curve->points) {
-        Vector3D d = p - centroid;
-        cov += d * d.transpose();
+    // Compute plane normal: use cross product of two spread directions, fall back to B face normal
+    Vector3D plane_n(0,0,0);
+    if (valid_curve->points.size() >= 3) {
+        Vector3D d1 = valid_curve->points[1] - valid_curve->points[0];
+        Vector3D d2 = valid_curve->points.back() - valid_curve->points[0];
+        plane_n = d1.cross(d2);
     }
-
-    Eigen::SelfAdjointEigenSolver<Eigen::Matrix3d> es(cov);
-    Vector3D plane_n = es.eigenvectors().col(0);  // smallest eigenvector = normal
-
     if (plane_n.norm() < 1e-12) {
-        // Degenerate: fall back to face normal of B
         plane_n = face_normal(body_b, face_b_id);
     }