Rebuild the geometry voxels after alignment

This fixes a crash of o2-sim in multithreaded Geant4 mode when the alignment is applied.

- TGeoPhysicalNode::Align only flags the voxels and assembly bounding boxes of the mother volumes for a rebuild.
- The rebuild then happens lazily during navigation, concurrently in several transport threads, and corrupts the heap.
- They are now rebuilt once at the end of MisalignGeometry, before transport starts.

https://its.cern.ch/jira/browse/O2-7305

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: sawenzel

Steer/src/O2MCApplication.cxx

@@ -46,6 +46,7 @@
 #include "DetectorsBase/GeometryManagerParam.h"
 #include <TGeoParallelWorld.h>
 #include <TGeoVolume.h>
+#include <TGeoVoxelFinder.h>
 #include <TBuffer3D.h>
 #include <TGeoTessellated.h>
 #include <DetectorsBase/O2Tessellated.h>
@@ -357,6 +358,24 @@ bool O2MCApplicationBase::MisalignGeometry()
   // performs possible optimizations (shape replacements on the runtime geometry)
   fixTGeoRuntimeShapes();
 
+  // alignment leaves voxels and assembly boxes to be rebuilt lazily during navigation, which
+  // is a data race between Geant4 transport threads; rebuild them here once
+  TIter nextvol(gGeoManager->GetListOfVolumes());
+  while (auto vol = static_cast<TGeoVolume*>(nextvol())) {
+    if (vol->IsAssembly()) {
+      vol->GetShape()->ComputeBBox();
+    }
+  }
+  nextvol.Reset();
+  int nvoxelized = 0;
+  while (auto vol = static_cast<TGeoVolume*>(nextvol())) {
+    if (auto voxels = vol->GetVoxels(); voxels && voxels->NeedRebuild()) {
+      voxels->Voxelize();
+      ++nvoxelized;
+    }
+  }
+  LOG(info) << "Rebuilt the voxels of " << nvoxelized << " volumes after alignment";
+
   // return original return value of misalignment procedure
   return true;
 }