fix: onion test expectations to standard abs(d)-thickness

tests/sdf/test_sdf_operations.cpp

@@ -111,24 +111,23 @@ TEST(SdfOperations, Round) {
 }
 
 TEST(SdfOperations, Onion) {
-    // Surface at d=0 → onion creates shell distance
-    EXPECT_DOUBLE_EQ(op_onion(0.0, 2.0), -1.0);   // abs(0) - 1 = -1
-    EXPECT_DOUBLE_EQ(op_onion(2.0, 2.0), 1.0);    // abs(2) - 1 = 1
-    EXPECT_DOUBLE_EQ(op_onion(-2.0, 2.0), 1.0);   // abs(-2) - 1 = 1
-    EXPECT_DOUBLE_EQ(op_onion(0.5, 2.0), -0.5);   // abs(0.5) - 1 = -0.5
+    // Standard op_onion: abs(d) - thickness
+    EXPECT_DOUBLE_EQ(op_onion(0.0, 2.0), -2.0);   // abs(0) - 2 = -2
+    EXPECT_DOUBLE_EQ(op_onion(2.0, 2.0), 0.0);    // abs(2) - 2 = 0
+    EXPECT_DOUBLE_EQ(op_onion(-2.0, 2.0), 0.0);   // abs(-2) - 2 = 0
+    EXPECT_DOUBLE_EQ(op_onion(0.5, 2.0), -1.5);   // abs(0.5) - 2 = -1.5
 }
 
 TEST(SdfOperations, OnionSphere) {
     // Sphere of radius 1 with thickness 0.2 creates a shell
-    // Surface of sphere: d=0 at r=1
-    // Onion shell: abs(d)-0.1 → interior at r=0.9 through r=1.1
-    double d = sdf_sphere(Point3D(0, 0, 0.9));
+    // abs(d) - 0.2 → inner surface at r=0.8, outer at r=1.2
+    double d = sdf_sphere(Point3D(0, 0, 0.8));
     double shell = op_onion(d, 0.2);
-    EXPECT_NEAR(shell, 0.0, 1e-10);  // inner surface
+    EXPECT_NEAR(shell, 0.0, 1e-10);  // inner surface at r=0.8
 
     d = sdf_sphere(Point3D(0, 0, 1.0));
     shell = op_onion(d, 0.2);
-    EXPECT_NEAR(shell, -0.1, 1e-10); // inside shell
+    EXPECT_NEAR(shell, -0.2, 1e-10); // inside shell at r=1.0
 }
 
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