add Spawning ntuple recording secondary births per step

Each step that spawns secondaries now writes one row per secondary to a
new Spawning ntuple (event_id, parent_track_id, parent_step_no,
child_track_id). This enables direct shower-tree reconstruction without
position joins. Requires the companion Geant4 submodule change that
pre-assigns track IDs in ProcessSecondariesFromParticleChange.

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

src/RunAction.cc

@@ -112,6 +112,14 @@ RunAction::RunAction()
   analysisManager->CreateNtupleDColumn("Ey");          // col 25 [V/m]
   analysisManager->CreateNtupleDColumn("Ez");          // col 26 [V/m]
   analysisManager->FinishNtuple();
+
+  // Spawning ntuple (one row per secondary born, ntuple id=2)
+  analysisManager->CreateNtuple("Spawning", "Spawning");
+  analysisManager->CreateNtupleIColumn("event_id");        // col 0
+  analysisManager->CreateNtupleIColumn("parent_track_id"); // col 1
+  analysisManager->CreateNtupleIColumn("parent_step_no");  // col 2
+  analysisManager->CreateNtupleIColumn("child_track_id");  // col 3
+  analysisManager->FinishNtuple();
 }
 
 //....oooOO0OOooo........oooOO0OOooo........oooOO0OOooo........oooOO0OOooo......

src/SteppingAction.cc

@@ -87,6 +87,18 @@ void SteppingAction::UserSteppingAction(const G4Step* step)
   auto track = step->GetTrack();
   int evtId  = G4RunManager::GetRunManager()->GetCurrentEvent()->GetEventID();
 
+  // Write one Spawning row per secondary born in this step (track IDs pre-assigned)
+  const auto* secondaries = step->GetSecondaryInCurrentStep();
+  if (secondaries) {
+    for (const G4Track* sec : *secondaries) {
+      analysisManager->FillNtupleIColumn(2, 0, evtId);
+      analysisManager->FillNtupleIColumn(2, 1, track->GetTrackID());
+      analysisManager->FillNtupleIColumn(2, 2, track->GetCurrentStepNumber());
+      analysisManager->FillNtupleIColumn(2, 3, sec->GetTrackID());
+      analysisManager->AddNtupleRow(2);
+    }
+  }
+
   analysisManager->FillNtupleIColumn(1, 0, evtId);
   analysisManager->FillNtupleIColumn(1, 1, track->GetTrackID());
   analysisManager->FillNtupleIColumn(1, 2, track->GetCurrentStepNumber());