Extract fake-shading math into pure ShadeColor

CityDoctorParent/Extensions/CityDoctorGUI/src/main/java/de/hft/stuttgart/citydoctor2/gui/gl/ShadeColor.java

+package de.hft.stuttgart.citydoctor2.gui.gl;
+
+import de.hft.stuttgart.citydoctor2.math.UnitVector3d;
+import de.hft.stuttgart.citydoctor2.math.Vector3d;
+import javafx.scene.paint.Color;
+
+/**
+ * Computes the surface-differentiation shade used by the legacy renderer: brightness derived from the
+ * angle between the polygon normal and a fixed axis. Not physical lighting; only makes adjacent faces
+ * distinguishable. Ported from {@code TriangulatedGeometry.calculateMaterial}.
+ */
+public final class ShadeColor {
+
+    private static final UnitVector3d AXIS = new Vector3d(19, 0.8, 1.5).normalize();
+
+    private ShadeColor() {}
+
+    public static Color shade(Vector3d normal, Color base) {
+        double cos = normal.dot(AXIS);
+        // clamp to acos domain; raw dot can drift slightly outside [-1,1] for near-parallel normals
+        cos = Math.max(-1.0, Math.min(1.0, cos));
+        double brightness = Math.acos(cos) / Math.PI * 0.6 + 0.3; // -> [0.3, 0.9]
+        return base.deriveColor(0, 1.0, brightness, 1.0);
+    }
+}

CityDoctorParent/Extensions/CityDoctorGUI/src/test/java/de/hft/stuttgart/citydoctor2/gui/gl/ShadeColorTest.java

+package de.hft.stuttgart.citydoctor2.gui.gl;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+import de.hft.stuttgart.citydoctor2.math.Vector3d;
+import javafx.scene.paint.Color;
+import org.junit.Test;
+
+public class ShadeColorTest {
+
+    @Test
+    public void brightnessStaysWithinExpectedBand() {
+        // Shade brightness is acos(normal.AXIS)/PI * 0.6 + 0.3 -> always within [0.3, 0.9].
+        Vector3d[] normals = {
+            new Vector3d(1, 0, 0), new Vector3d(0, 1, 0), new Vector3d(0, 0, 1),
+            new Vector3d(-1, 0, 0), new Vector3d(0.3, -0.7, 0.5)
+        };
+        for (Vector3d n : normals) {
+            Color c = ShadeColor.shade(n.normalize(), Color.WHITE);
+            assertTrue("brightness >= 0.3", c.getBrightness() >= 0.3 - 1e-6);
+            assertTrue("brightness <= 0.9", c.getBrightness() <= 0.9 + 1e-6);
+        }
+    }
+
+    @Test
+    public void matchesLegacyFormulaForKnownNormal() {
+        Vector3d axis = new Vector3d(19, 0.8, 1.5).normalize();
+        Vector3d normal = new Vector3d(0, 0, 1).normalize();
+        double cos = Math.max(-1.0, Math.min(1.0, normal.dot(axis)));
+        double shade = Math.acos(cos) / Math.PI * 0.6 + 0.3;
+        Color expected = Color.WHITE.deriveColor(0, 1.0, shade, 1.0);
+        Color actual = ShadeColor.shade(normal, Color.WHITE);
+        assertEquals(expected.getRed(),   actual.getRed(),   1e-9);
+        assertEquals(expected.getGreen(), actual.getGreen(), 1e-9);
+        assertEquals(expected.getBlue(),  actual.getBlue(),  1e-9);
+    }
+}