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/Measurement.h

@@ -38,6 +38,15 @@
 double volumeOfObjects(ViewportWindow& vp,
                        const std::vector<uint32_t>& object_ids);
 
+// Per-object volumes (m³).  Same algorithm as volumeOfObjects but
+// attributed per id rather than summed.  Skips ids that don't resolve
+// to a live instance, so the result may be shorter than the input.
+// Used by MainWindow's volume readout to drive both the total HUD and
+// the per-object overlay labels.
+std::vector<std::pair<uint32_t, double>>
+volumesPerObject(ViewportWindow& vp,
+                 const std::vector<uint32_t>& object_ids);
+
 // Click-to-accumulate area measurement.  Each pick resolves the screen
 // click to a (instance, triangle) using ViewportWindow's primitives,
 // expands it into the connected coplanar patch (BFS over shared edges,