add per-step kinematics ntuple ("Steps") to ROOT output

Adds a second ntuple alongside the existing "Hits" ntuple with one row
per Geant4 step. Records event_id, track_id, step_no, PDG code, and
pre/post position + kinetic energy for every step in the simulation.

Also extends run_batch() with return_steps=False; when True, returns
(hits_mf, steps_mf) instead of just hits_mf.

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

bind/G4Calo.py

@@ -334,7 +334,8 @@ def run_batch(
         maxEnergy_GeV: float = -1.0,
         filename: str = "",
         no_mp: bool = False,
-        manual_seed : int = -1):
+        manual_seed : int = -1,
+        return_steps: bool = False):
     """
 Run a full Geant4 simulation batch with automatic parallelisation.
 
@@ -361,12 +362,17 @@ no_mp : bool, optional
     Disable multiprocessing (run single-threaded) for debugging.
 manual_seed : int, optional
     Optional random seed. If < 0, seeds are generated automatically.
+return_steps : bool, optional
+    If True, also return a MiniFrame of step-level data (one row per
+    Geant4 step across all events). Returns a tuple (hits, steps).
 
 Returns
 -------
-MiniFrame
-    Table-like structure with one row per event, containing both scalar
-    values and fixed-size sensor arrays.
+MiniFrame or tuple[MiniFrame, MiniFrame]
+    If return_steps is False (default): event-level MiniFrame with one
+    row per event. If return_steps is True: (hits, steps) where steps
+    has one row per Geant4 step with columns event_id, track_id,
+    step_no, pdg, pre_x/y/z, pre_E, post_x/y/z, post_E.
 
 Notes
 -----
@@ -463,6 +469,9 @@ Example
             if os.path.exists(f):
                 os.remove(f)
     print('G4Calo: concatenation finished after {:.2f} seconds'.format(sw.elapsed()))
+    if return_steps:
+        steps_df = _assemble_results_to_mini_df(rp, tree_name="Steps")
+        return df, steps_df
     return df
 
 def _fill_event(gd : GeometryDescriptor,