ifcviewer: move volume readout helpers into ViewportCore (#84-j)

Tiny followup to #84-i — move the const-lookup volume helpers used by
bonsai's measurement HUD:

  double volumeOfObjects(const std::vector<uint32_t>&) const
  vector<pair<uint32_t, double>> volumesPerObject(
                              const std::vector<uint32_t>&) const

The det3OfPlacement static helper moves with them into ViewportCore.cpp's
anonymous namespace (the original kept its mirror in
ViewportWindow.cpp; ViewportWindow's own internal callers are gone now
since these methods moved).

Pure read of models_gpu_ + mesh_local_volumes — all in core already.
Trivial transplant.

Builds: desktop / bonsai / web all green. Tests 100/100.

src/ifcviewer/ViewportCore.h

@@ -40,6 +40,8 @@
 #include <cstdint>
 #include <string>
 #include <unordered_map>
+#include <utility>
+#include <vector>
 
 #include "BufferPool.h"
 #include "InstanceCompose.h"
@@ -158,6 +160,17 @@ public:
     bool computeObjectAabb(uint32_t object_id,
                            Eigen::Vector3f& mn, Eigen::Vector3f& mx) const;
 
+    // Sum of mesh-local volumes (m³) of every instance whose object_id
+    // is in `object_ids`. Each instance is scaled by |det(placement_3x3)|
+    // to pick up mapped-item scale/mirror; signed-tetrahedra absolute
+    // value means winding is ignored. Volumes are precomputed at
+    // applyCachedModel — this call is just lookups + multiplies.
+    double volumeOfObjects(const std::vector<uint32_t>& object_ids) const;
+    // Per-object variant. Used by the Volume tool to drive both the
+    // total HUD and the per-object overlay labels at AABB centres.
+    std::vector<std::pair<uint32_t, double>>
+        volumesPerObject(const std::vector<uint32_t>& object_ids) const;
+
     // Friend access for ViewportWindow's reference proxies. As each
     // render method moves into ViewportCore it stops needing these
     // (it touches the fields directly); once everything has migrated