Add SceneData container for built scene buffers

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

+package de.hft.stuttgart.citydoctor2.gui.gl;
+
+import java.util.List;
+
+/** Immutable CPU-side result of building the scene; uploaded to the GPU by the GL layer. */
+public class SceneData {
+
+    private final float[] positions;
+    private final float[] colors;
+    private final int[] ids;
+    private final List<TriangleMeta> triangleMetas;
+    private final PickRegistry pickRegistry;
+    private final double[] center;
+
+    public SceneData(float[] positions, float[] colors, int[] ids,
+                     List<TriangleMeta> triangleMetas, PickRegistry pickRegistry, double[] center) {
+        this.positions = positions;
+        this.colors = colors;
+        this.ids = ids;
+        this.triangleMetas = triangleMetas;
+        this.pickRegistry = pickRegistry;
+        this.center = center;
+    }
+
+    public float[] positions() { return positions; }
+    public float[] colors() { return colors; }
+    public int[] ids() { return ids; }
+    public List<TriangleMeta> triangleMetas() { return triangleMetas; }
+    public PickRegistry pickRegistry() { return pickRegistry; }
+    public double[] center() { return center; }
+    public int triangleCount() { return triangleMetas.size(); }
+}

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

+package de.hft.stuttgart.citydoctor2.gui.gl;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertSame;
+import java.util.List;
+import org.junit.Test;
+
+public class SceneDataTest {
+
+    @Test
+    public void exposesItsBuffersAndRegistry() {
+        float[] pos = {0,0,0, 1,0,0, 0,1,0};
+        float[] col = {1,1,1, 1,1,1, 1,1,1};
+        int[] ids = {1,1,1};
+        PickRegistry reg = new PickRegistry();
+        List<TriangleMeta> metas = List.of(new TriangleMeta(0, null, 2, false));
+        SceneData data = new SceneData(pos, col, ids, metas, reg, new double[]{5, 6, 7});
+
+        assertSame(pos, data.positions());
+        assertEquals(1, data.triangleMetas().size());
+        assertEquals(6.0, data.center()[1], 0.0);
+        assertSame(reg, data.pickRegistry());
+    }
+}