Add next_volume/next_material columns and sampling batch script

Records the volume and material a track is about to enter when a step
ends at a geometric boundary, since post-step's physical volume still
refers to the volume the step occurred in. Also adds
run-sampling-batch.sh to run all run_sampling configs in parallel from
isolated directories, avoiding output filename collisions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/RunAction.cc

@@ -114,6 +114,8 @@ RunAction::RunAction()
   analysisManager->CreateNtupleDColumn("post_dx");     // col 27
   analysisManager->CreateNtupleDColumn("post_dy");     // col 28
   analysisManager->CreateNtupleDColumn("post_dz");     // col 29
+  analysisManager->CreateNtupleSColumn("next_volume");   // col 30 (volume entered if step ended at a boundary, else "")
+  analysisManager->CreateNtupleSColumn("next_material"); // col 31 (material entered if step ended at a boundary, else "")
   analysisManager->FinishNtuple();
 
   // Spawning ntuple (one row per secondary born, ntuple id=2)

src/SteppingAction.cc

@@ -149,6 +149,22 @@ void SteppingAction::UserSteppingAction(const G4Step* step)
   analysisManager->FillNtupleDColumn(1, 28, postDir.y());
   analysisManager->FillNtupleDColumn(1, 29, postDir.z());
 
+  // H: volume/material the track is about to enter, if this step ended at a
+  // geometric boundary. post->GetPhysicalVolume() still refers to the volume
+  // the step occurred in, not the one being entered, so use track->GetNextVolume().
+  G4String nextVolumeName = "";
+  G4String nextMaterialName = "";
+  if (post->GetStepStatus() == fGeomBoundary) {
+    auto nextVolume = track->GetNextVolume();
+    if (nextVolume) {
+      nextVolumeName = nextVolume->GetName();
+      auto nextMaterial = nextVolume->GetLogicalVolume()->GetMaterial();
+      if (nextMaterial) nextMaterialName = nextMaterial->GetName();
+    }
+  }
+  analysisManager->FillNtupleSColumn(1, 30, nextVolumeName);
+  analysisManager->FillNtupleSColumn(1, 31, nextMaterialName);
+
   // Field: B [T] and E [V/m] at pre-step position; zero if no field is registered
   G4double Bx=0, By=0, Bz=0, Ex=0, Ey=0, Ez=0;
   const auto* fm = G4TransportationManager::GetTransportationManager()->GetFieldManager();