add child_track_ids array column to Steps; remove parent_id

Each Steps row now carries a variable-length child_track_ids int array
containing the track IDs of all secondaries spawned during that step.
The parent_id column is removed — parentage is now expressed
unidirectionally (parent→children) via child_track_ids and the Spawning
ntuple, rather than bidirectionally.

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

include/RunAction.hh

@@ -83,6 +83,8 @@ class RunAction : public G4UserRunAction
   mutable std::vector<int> hitLayer;
   mutable std::vector<int> hitCopyNumber;
 
+  mutable std::vector<int> stepChildIds;
+
   mutable G4String filename;
 
 };

include/SteppingAction.hh

@@ -33,6 +33,7 @@
 #include "G4UserSteppingAction.hh"
 #include "GeometryDescriptor.hh"
 #include "DetectorConstruction.hh"
+#include "RunAction.hh"
 
 
 namespace B4a
@@ -54,8 +55,11 @@ public:
 
   void UserSteppingAction(const G4Step* step) override;
 
+  void setRunAction(const B4::RunAction* runAction) { fRunAction = runAction; }
+
 private:
   EventAction* fEventAction = nullptr;
+  const B4::RunAction* fRunAction = nullptr;
 };
 
 }