Add TriangleMeta and IndexSetBuilder for filter-driven draw sets

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

+package de.hft.stuttgart.citydoctor2.gui.gl;
+
+import java.util.List;
+import java.util.function.Predicate;
+
+/** Builds the element index set (triangle vertex indices) for the triangles passing the filter. */
+public final class IndexSetBuilder {
+
+    private IndexSetBuilder() {}
+
+    public static int[] build(List<TriangleMeta> metas, Predicate<TriangleMeta> include) {
+        IntList out = new IntList();
+        for (TriangleMeta m : metas) {
+            if (include.test(m)) {
+                int base = m.triangleIndex() * 3;
+                out.add(base);
+                out.add(base + 1);
+                out.add(base + 2);
+            }
+        }
+        return out.toArray();
+    }
+}

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

+package de.hft.stuttgart.citydoctor2.gui.gl;
+
+import de.hft.stuttgart.citydoctor2.datastructure.Polygon;
+
+/**
+ * Metadata for one triangle in the global buffer.
+ * @param triangleIndex index of this triangle (vertices are 3*triangleIndex .. +2)
+ * @param polygon       source polygon (for picking/highlight resolution); may be null in tests
+ * @param lod           LOD ordinal (Lod.ordinal()) of the owning geometry
+ * @param roof          true if the triangle belongs to a ROOF boundary surface
+ */
+public record TriangleMeta(int triangleIndex, Polygon polygon, int lod, boolean roof) {}

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

+package de.hft.stuttgart.citydoctor2.gui.gl;
+
+import static org.junit.Assert.assertArrayEquals;
+import java.util.Arrays;
+import java.util.List;
+import java.util.function.Predicate;
+import org.junit.Test;
+
+public class IndexSetBuilderTest {
+
+    @Test
+    public void includesOnlyTrianglesMatchingThePredicate() {
+        List<TriangleMeta> metas = Arrays.asList(
+                new TriangleMeta(0, null, 1, false),
+                new TriangleMeta(1, null, 2, false),
+                new TriangleMeta(2, null, 1, false));
+        Predicate<TriangleMeta> onlyGroup1 = m -> m.lod() == 1;
+        int[] indices = IndexSetBuilder.build(metas, onlyGroup1);
+        assertArrayEquals(new int[]{0, 1, 2, 6, 7, 8}, indices);
+    }
+
+    @Test
+    public void emptyWhenNothingMatches() {
+        List<TriangleMeta> metas = Arrays.asList(new TriangleMeta(0, null, 1, false));
+        int[] indices = IndexSetBuilder.build(metas, m -> false);
+        assertArrayEquals(new int[]{}, indices);
+    }
+}