ifcviewer-full: print object volume on click via lazy GPU readback

Adds neutral primitives on ViewportWindow (readbackMeshTriangles,
findInstance) so consumers can compute per-object geometry queries
without the library retaining a CPU triangle copy. Measurement.cpp in
ifcviewer-full uses them to sum signed-tetrahedra in mesh-local space,
weighted by |det(placement_3x3)| per instance for mapped-item scaling.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ifcviewer-full/Measurement.h

@@ -0,0 +1,38 @@
+/********************************************************************************
+ *                                                                              *
+ * This file is part of IfcOpenShell.                                           *
+ *                                                                              *
+ * IfcOpenShell is free software: you can redistribute it and/or modify         *
+ * it under the terms of the Lesser GNU General Public License as published by  *
+ * the Free Software Foundation, either version 3.0 of the License, or          *
+ * (at your option) any later version.                                          *
+ *                                                                              *
+ * IfcOpenShell is distributed in the hope that it will be useful,              *
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of               *
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the                 *
+ * Lesser GNU General Public License for more details.                          *
+ *                                                                              *
+ * You should have received a copy of the Lesser GNU General Public License     *
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.         *
+ *                                                                              *
+ ********************************************************************************/
+
+#ifndef IFCVIEWER_FULL_MEASUREMENT_H
+#define IFCVIEWER_FULL_MEASUREMENT_H
+
+#include <cstdint>
+#include <vector>
+
+class ViewportWindow;
+
+// Sum of mesh-local volumes (m³) of every instance whose object_id is in
+// `object_ids`.  Groups by (model, mesh) so each unique mesh is read back
+// from the GPU at most once per call; instances of the same mesh are scaled
+// by |det(placement_3x3)| to pick up mapped-item scale/mirror.  Volume is
+// taken as the absolute value of the signed-tetrahedra sum, so winding
+// convention does not matter.  Returns 0.0 for empty input or when nothing
+// resolves.  Recomputes from scratch on every call — no cache.
+double volumeOfObjects(ViewportWindow& vp,
+                       const std::vector<uint32_t>& object_ids);
+
+#endif // IFCVIEWER_FULL_MEASUREMENT_H