Add PickRegistry mapping picking ids to model objects

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

+package de.hft.stuttgart.citydoctor2.gui.gl;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/** Maps picking ids (1-based) to the model objects they represent. */
+public class PickRegistry {
+
+    private final List<Object> targets = new ArrayList<>();
+
+    /** Registers a target and returns its non-zero picking id. */
+    public int register(Object target) {
+        targets.add(target);
+        return targets.size(); // id = index + 1
+    }
+
+    /** Resolves a picking id to its target, or null for background / unknown id. */
+    public Object resolve(int id) {
+        if (id <= 0 || id > targets.size()) {
+            return null;
+        }
+        return targets.get(id - 1);
+    }
+
+    public int size() { return targets.size(); }
+}

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

+package de.hft.stuttgart.citydoctor2.gui.gl;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
+import org.junit.Test;
+
+public class PickRegistryTest {
+
+    @Test
+    public void assignsSequentialNonZeroIdsAndResolvesThem() {
+        PickRegistry reg = new PickRegistry();
+        Object a = "a";
+        Object b = "b";
+        int idA = reg.register(a);
+        int idB = reg.register(b);
+        assertEquals(1, idA);
+        assertEquals(2, idB);
+        assertEquals(a, reg.resolve(idA));
+        assertEquals(b, reg.resolve(idB));
+    }
+
+    @Test
+    public void resolvesBackgroundAndOutOfRangeToNull() {
+        PickRegistry reg = new PickRegistry();
+        reg.register("a");
+        assertNull(reg.resolve(0));
+        assertNull(reg.resolve(99));
+    }
+}