fix: step_import — use find/insert_or_assign for unordered_map of non-default-constructible types

src/brep/step_import.cpp

@@ -1225,7 +1225,8 @@ private:
     }
 
     curves::NurbsCurve get_or_build_curve(int ref) {
-        if (curve_cache_.count(ref)) return curve_cache_[ref];
+        auto cit = curve_cache_.find(ref);
+        if (cit != curve_cache_.end()) return cit->second;
         if (!entities_.count(ref)) {
             // Return a dummy curve
             return curves::NurbsCurve({Point3D(0,0,0), Point3D(1,0,0)}, {0,0,1,1}, {1,1}, 1);
@@ -1251,12 +1252,13 @@ private:
             set_error(StepError::EntityTypeError, "Error building curve #" + std::to_string(ref) + ": " + e.what());
             nc = curves::NurbsCurve({Point3D(0,0,0), Point3D(1,0,0)}, {0,0,1,1}, {1,1}, 1);
         }
-        curve_cache_[ref] = nc;
+        curve_cache_.insert_or_assign(ref, nc);
         return nc;
     }
 
     curves::NurbsSurface get_or_build_surface(int ref) {
-        if (surface_cache_.count(ref)) return surface_cache_[ref];
+        auto sit = surface_cache_.find(ref);
+        if (sit != surface_cache_.end()) return sit->second;
         if (!entities_.count(ref)) {
             // Return a dummy surface
             std::vector<std::vector<Point3D>> g = {{Point3D(0,0,0), Point3D(1,0,0)},
@@ -1293,7 +1295,7 @@ private:
                                                     {Point3D(0,1,0), Point3D(1,1,0)}};
             ns = curves::NurbsSurface(g, {0,0,1,1}, {0,0,1,1}, {}, 1, 1);
         }
-        surface_cache_[ref] = ns;
+        surface_cache_.insert_or_assign(ref, ns);
         return ns;
     }