ifcviewer-full: volume tool with HUD + per-object labels

Mirrors the Area tool's display: HUD shows total volume + object count,
each selected object gets a label at its world-AABB centroid showing
its individual volume.  Gated behind ToolMode::Volume (Ctrl+Shift+V) so
it stays out of the way until invoked.

Volume is a passive tool — selection works as in None (multi-select,
modifier toggle, box-select all keep working).  Area / Length still
intercept clicks through surfacePickedInTool.

Adds volumesPerObject() reusing the same mesh-cached readback path as
volumeOfObjects, so the per-object split costs no extra GL readbacks.
computeObjectAabb is promoted to public for the centroid lookup.

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

src/ifcviewer-full/MainWindow.h

@@ -93,6 +93,10 @@ private:
     bool confirmDiscardIfDirty();
     void updateWindowTitle();
     void populateProperties(uint32_t object_id);
+    // Push the volume HUD + per-object volume labels for the current
+    // selection.  No-op when a measurement tool is active — that tool
+    // owns the overlay state until it's exited.
+    void updateVolumeReadout();
     void appendElementToTree(uint32_t model_id,
                              uint32_t object_id,
                              int ifc_id,