fix: onion thickness — reapply correct formula

include/vde/sdf/sdf_operations.h

@@ -47,7 +47,7 @@ using core::Vector3D;
 }
 
 [[nodiscard]] inline double op_onion(double d, double thickness) {
-    return std::abs(d) - thickness * 0.5;
+    return std::abs(d) - thickness;
 }
 
 // ── Domain deformation operators (transform point space) ──

tests/sdf/test_sdf_operations.cpp

@@ -378,8 +378,7 @@ TEST(SdfOperations, CompositionDifferenceWithTwist) {
 
     // At origin, both spheres contain the point
     // body: sqrt(0) - 2 = -2; cutter: sqrt(0) - 1 = -1
-    // difference = max(-(-2), -1) = max(2, -1) = 2 → outside!
-    // Wait: difference = max(-d1, d2). So max(-(-2), -1) = max(2, -1) = 2
+    // difference = max(-2, -(-1)) = max(-2, 1) = 1 → outside!
     EXPECT_GT(scene(Point3D(0, 0, 0)), 0.0);
 
     // At r=1.5 from origin: body=-0.5, cutter=0.5 (at y=0 twist=0 → sphere at r=1)