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();