pre-assign secondary track IDs in ProcessSecondariesFromParticleChange

Add G4EventManager::ReserveNextTrackID() to atomically increment and
return the next track ID counter. Call it from G4SteppingManager::
ProcessSecondariesFromParticleChange() so that each secondary has its
track ID set before UserSteppingAction fires. G4EventManager::StackTracks
is updated to skip re-assignment when GetTrackID() != 0 (pre-assigned).

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

source/event/include/G4EventManager.hh

@@ -59,6 +59,10 @@ class G4EventManager
     static G4EventManager* GetEventManager();
       // This method returns the singleton pointer of G4EventManager.
 
+    G4int ReserveNextTrackID();
+      // Pre-assign the next sequential track ID to a secondary being born inside
+      // G4SteppingManager, so that its ID is available in UserSteppingAction.
+
     G4EventManager();
    ~G4EventManager();
 

source/event/src/G4EventManager.cc

@@ -401,6 +401,11 @@ G4int G4EventManager::StoreSubEvent(G4Event* evt, G4int& subEvtType, G4SubEvent*
   return evt->StoreSubEvent(subEvtType,se);
 }
 
+G4int G4EventManager::ReserveNextTrackID()
+{
+  return ++trackIDCounter;
+}
+
 void G4EventManager::StackTracks(G4TrackVector* trackVector,
                                  G4bool IDhasAlreadySet)
 {
@@ -409,9 +414,13 @@ void G4EventManager::StackTracks(G4TrackVector* trackVector,
     if( trackVector->empty() ) return;
     for( auto newTrack : *trackVector )
     {
-      ++trackIDCounter;
-      if(!IDhasAlreadySet)
+      if(IDhasAlreadySet)
       {
+        ++trackIDCounter; // advance counter for primaries whose IDs are set externally
+      }
+      else if(newTrack->GetTrackID() == 0)
+      {
+        ++trackIDCounter; // normal secondary: assign as usual
         newTrack->SetTrackID( trackIDCounter );
         if(newTrack->GetDynamicParticle()->GetPrimaryParticle() != nullptr)
         {
@@ -420,6 +429,7 @@ void G4EventManager::StackTracks(G4TrackVector* trackVector,
           pp->SetTrackID(trackIDCounter);
         }
       }
+      // else: ID was pre-assigned by ReserveNextTrackID() in G4SteppingManager — skip
       newTrack->SetOriginTouchableHandle(newTrack->GetTouchableHandle());
       trackContainer->PushOneTrack( newTrack );
 #ifdef G4VERBOSE