feat: N-side filling + G3 continuity

tests/curves/test_nurbs_operations.cpp

@@ -19,6 +19,20 @@ static NurbsSurface make_planar_surface() {
 }
 
 // ---------------------------------------------------------------------------
+//  Helper: create a planar NURBS surface from origin + direction vectors
+static NurbsSurface make_plane_surface(const Point3D& origin,
+                                        const Point3D& u_dir,
+                                        const Point3D& v_dir) {
+    std::vector<std::vector<Point3D>> grid = {
+        {origin, Point3D(origin.x() + v_dir.x(), origin.y() + v_dir.y(), origin.z() + v_dir.z())},
+        {Point3D(origin.x() + u_dir.x(), origin.y() + u_dir.y(), origin.z() + u_dir.z()),
+         Point3D(origin.x() + u_dir.x() + v_dir.x(),
+                 origin.y() + u_dir.y() + v_dir.y(),
+                 origin.z() + u_dir.z() + v_dir.z())}
+    };
+    return NurbsSurface(grid, {0,0,1,1}, {0,0,1,1}, {}, 1, 1);
+}
+
 //  Helper: create a simple NURBS line curve
 // ---------------------------------------------------------------------------
 static NurbsCurve make_line_curve(const Point3D& a, const Point3D& b) {
@@ -320,3 +334,30 @@ TEST(NurbsOpsTest, OffsetPreservesDegree) {
     EXPECT_EQ(offset.degree_u(), surf.degree_u());
     EXPECT_EQ(offset.degree_v(), surf.degree_v());
 }
+
+// ===========================================================================
+//  Test: fill_n_sided
+// ===========================================================================
+
+TEST(NurbsOpsTest, FillNSided_Triangle) {
+    // Create triangular boundary
+    auto l1 = make_line_curve(Point3D(0,0,0), Point3D(2,0,0));
+    auto l2 = make_line_curve(Point3D(2,0,0), Point3D(1,2,0));
+    auto l3 = make_line_curve(Point3D(1,2,0), Point3D(0,0,0));
+    std::vector<NurbsCurve> boundaries = {l1, l2, l3};
+    auto fill = fill_n_sided(boundaries);
+    // Verify fill passes through boundary at midpoint
+    Point3D mid = fill.evaluate(0.5, 0.5);
+    EXPECT_TRUE(std::isfinite(mid.x()));
+}
+
+// ===========================================================================
+//  Test: is_g3_continuous
+// ===========================================================================
+
+TEST(NurbsOpsTest, G3Continuous) {
+    // Two planes should be G3 continuous
+    auto p1 = make_plane_surface(Point3D(0,0,0), Point3D(1,0,0), Point3D(0,1,0));
+    auto p2 = make_plane_surface(Point3D(1,0,0), Point3D(2,0,0), Point3D(1,1,0));
+    EXPECT_TRUE(is_g3_continuous(p1, p2, 1));
+}