tests: port HyperIdealVisualizationPlugin circle-projection tests

Implements getEuclideanCircleFromHyperbolic() in C++ (hyperboloid model
→ Poincaré disk via Lorentz boost + circumcircle). Ports the 2 Java tests
that were the only remaining candidates not requiring HDS or a solver.

All other unported tests (DataTypesTest, SchottkyIOTest,
UniformizationDataTest, BranchedCoverTorusTest) are blocked by XML
serialization or HDS – stubs remain for Phase 4.

Test totals: 36 registered | 23 passed | 13 skipped | 0 failed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

code/tests/test_hyper_ideal_visualization_utility.cpp

@@ -0,0 +1,50 @@
+// Port of de.varylab.discreteconformal.plugin.HyperIdealVisualizationPluginTest (Java/JUnit).
+//
+// Tests the conversion from a hyperbolic circle (hyperboloid model)
+// to its Euclidean representation (Poincaré disk model).
+//
+// Java test: static method HyperIdealVisualizationPlugin
+//              .getEuclideanCircleFromHyperbolic(double[] center, double radius)
+// C++ port:  conformallab::getEuclideanCircleFromHyperbolic(Vector4d, double)
+//              in hyper_ideal_visualization_utility.hpp
+
+#include "hyper_ideal_visualization_utility.hpp"
+#include <gtest/gtest.h>
+#include <cmath>
+
+using namespace conformallab;
+
+// Corresponds to Java testGetEuclideanCircleFromHyperbolic_Centered()
+//
+// A hyperbolic circle centered at the origin (0,0,0,1) with radius 1.
+// In the Poincaré disk this maps to a Euclidean circle centered at (0,0)
+// with radius  sinh(1) / (cosh(1) + 1).
+TEST(HyperIdealVisualizationUtilityTest, EuclideanCircleFromHyperbolic_Centered)
+{
+    Eigen::Vector4d center(0.0, 0.0, 0.0, 1.0);
+    const double radius = 1.0;
+
+    auto result = getEuclideanCircleFromHyperbolic(center, radius);
+
+    const double expected_r = std::sinh(1.0) / (std::cosh(1.0) + 1.0);
+    EXPECT_NEAR(0.0,        result[0], 1E-12) << "Euclidean cx should be 0";
+    EXPECT_NEAR(0.0,        result[1], 1E-12) << "Euclidean cy should be 0";
+    EXPECT_NEAR(expected_r, result[2], 1E-12) << "Euclidean radius mismatch";
+}
+
+// Corresponds to Java testGetEuclideanCircleFromHyperbolic_OffCenter()
+//
+// A hyperbolic circle centered at (sinh(1),0,0,cosh(1)) with radius 1.
+// By symmetry (center is on the x-axis, circle is symmetric about it):
+//   • the Euclidean center lies on the x-axis  → cy = 0
+//   • the Euclidean center equals the radius   → cx = r  (the circle passes through the Poincaré origin)
+TEST(HyperIdealVisualizationUtilityTest, EuclideanCircleFromHyperbolic_OffCenter)
+{
+    Eigen::Vector4d center(std::sinh(1.0), 0.0, 0.0, std::cosh(1.0));
+    const double radius = 1.0;
+
+    auto result = getEuclideanCircleFromHyperbolic(center, radius);
+
+    EXPECT_NEAR(result[0], result[2], 1E-12) << "cx should equal Euclidean radius";
+    EXPECT_NEAR(0.0, result[1],       1E-12) << "cy should be 0 (x-axis symmetry)";
+}