Add PickingCodec for color-picking id encoding

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

+package de.hft.stuttgart.citydoctor2.gui.gl;
+
+/** Encodes integer ids into RGB bytes for GPU color-picking. Id 0 is reserved for background. */
+public final class PickingCodec {
+
+    private PickingCodec() {}
+
+    public static int red(int id)   { return id & 0xFF; }
+    public static int green(int id) { return (id >> 8) & 0xFF; }
+    public static int blue(int id)  { return (id >> 16) & 0xFF; }
+
+    public static int decode(int r, int g, int b) {
+        return (r & 0xFF) | ((g & 0xFF) << 8) | ((b & 0xFF) << 16);
+    }
+}

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

+package de.hft.stuttgart.citydoctor2.gui.gl;
+
+import static org.junit.Assert.assertEquals;
+import org.junit.Test;
+
+public class PickingCodecTest {
+
+    @Test
+    public void roundTripsIdsThroughRgbBytes() {
+        int[] ids = {1, 255, 256, 65535, 65536, 16777215};
+        for (int id : ids) {
+            int r = PickingCodec.red(id);
+            int g = PickingCodec.green(id);
+            int b = PickingCodec.blue(id);
+            assertEquals(id, PickingCodec.decode(r, g, b));
+        }
+    }
+
+    @Test
+    public void blackDecodesToZeroBackground() {
+        assertEquals(0, PickingCodec.decode(0, 0, 0));
+    }
+}