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/tracking/src/G4SteppingManager.cc

@@ -32,6 +32,7 @@
 // --------------------------------------------------------------------
 
 #include "G4SteppingManager.hh"
+#include "G4EventManager.hh"
 
 #include "G4ForceCondition.hh"
 #include "G4GPILSelection.hh"
@@ -600,6 +601,10 @@ G4int G4SteppingManager::ProcessSecondariesFromParticleChange()
     // Set the process pointer which created this track
     tempSecondaryTrack->SetCreatorProcess(creatorProcess);
 
+    // Pre-assign track ID so it is visible in UserSteppingAction via
+    // GetSecondaryInCurrentStep(); G4EventManager::StackTracks skips re-assignment.
+    tempSecondaryTrack->SetTrackID(G4EventManager::GetEventManager()->ReserveNextTrackID());
+
     // If this 2ndry particle has 'zero' kinetic energy, make sure
     // it invokes a rest process at the beginning of the tracking
     //