fix: SDF nan/edge cases + B-Rep vertex index → ID fix + IGES 1-based DE + STEP cleanup

tests/sdf/test_sdf_primitives.cpp

@@ -282,7 +282,7 @@ TEST(SdfEllipsoid, OnSurface) {
 TEST(SdfTriangularPrism, OnVertex) {
     Point3D a(0, 0, 0), b(4, 0, 0), c(2, 3, 0);
     double d = triangular_prism(Point3D(0, 0, 0), a, b, c, 2.0);
-    EXPECT_NEAR(d, -1.0, 1e-6);
+    EXPECT_NEAR(d, 0.0, 1e-6);  // vertex on boundary → SDF = 0
 }
 
 TEST(SdfTriangularPrism, Inside) {
@@ -344,10 +344,12 @@ TEST(SdfLink, AtCenterOfFirstTorus) {
 }
 
 TEST(SdfLink, BetweenToruses) {
-    // Point between the two toruses, inside the overlap region
+    // Point between the two toruses — this is in the torus hole, outside the shape
     double d = link(Point3D(0, 0, 0), 2.0, 2.0, 0.5);
-    // This is inside both toruses, so negative
-    EXPECT_LT(d, 0.0);
+    // Point (0,0,0) is in the hole region between the toruses
+    // With length=2 (centers at ±1), major_r=2, minor_r=0.5:
+    // distance to each torus tube ≈ 0.5, so the point is outside
+    EXPECT_GT(d, 0.0);
 }
 
 TEST(SdfLink, FarOutside) {