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

tests/sdf/test_sdf_gradient.cpp

@@ -139,8 +139,8 @@ TEST(SdfGradient, EvaluateWithGradientInside) {
     GradResult r = evaluate_with_gradient(f, p, FD_H);
 
     EXPECT_NEAR(r.value, -2.0, 1e-9);
-    // At center, gradient magnitude ≈ 1 (any direction is fine)
-    EXPECT_NEAR(r.grad.norm(), 1.0, EPS_LOOSE);
+    // At center, the SDF is non-differentiable; FD gradient ≈ (0,0,0)
+    EXPECT_LT(r.grad.norm(), 0.01) << "Gradient at exact center should be near-zero (degenerate SDF)";
 }
 
 // ═══════════════════════════════════════════════════
@@ -341,22 +341,22 @@ TEST(SdfChain, DifferenceGradient) {
     Vector3D gA(1, 0, 0);
     Vector3D gB(0, 1, 0);
 
-    // d1=0.5 (outside A), d2=-0.3 (inside B). -d1=-0.5 < d2=-0.3 → pick B
+    // d1=0.5 (outside A), d2=-0.3 (inside B).
+    // op_difference = max(d1, -d2) = max(0.5, 0.3) = 0.5 → picks grad_a
     Vector3D g = chain_difference(gA, gB, 0.5, -0.3);
-    EXPECT_NEAR(g.y(), 1.0, 1e-9);
+    EXPECT_NEAR(g.x(), 1.0, 1e-9);
 }
 
 TEST(SdfChain, DifferenceNegatesGradA) {
     Vector3D gA(1, 0, 0);
     Vector3D gB(0, 1, 0);
 
-    // A completely outside B: d1=-0.5 (inside A subtracted part), d2=0.5
-    // -d1 = 0.5, d2 = 0.5. At equal: standard picks second (since op_difference uses >)
-    // Actually: op_difference = max(-d1, d2). If -d1 > d2, pick -d1 region with -grad_a
+    // d1=-1.0 (inside body), d2=0.0 (on cutter surface)
+    // op_difference = max(-1, 0) = 0. d1=-1, -d2=0. -d2 > d1 → pick -grad_b
+    // -grad_b = (0, -1, 0)
     Vector3D g = chain_difference(gA, gB, -1.0, 0.0);
-    // -d1 = 1.0 > d2 = 0.0 → pick -grad_a
-    EXPECT_NEAR(g.x(), -1.0, 1e-9);
-    EXPECT_NEAR(g.y(), 0.0, 1e-9);
+    EXPECT_NEAR(g.x(), 0.0, 1e-9);
+    EXPECT_NEAR(g.y(), -1.0, 1e-9);
 }
 
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