Encode edep/secondary/post energy as a conservation-constrained simplex

Replaces the independent log_delta_e/log_edep targets with 2 additive-log-ratio
coordinates over the deposit/secondary/post-energy simplex (fractions of pre_E
summing to 1), so edep + e_sec + post_E == pre_E holds by construction after
decoding (softmax) rather than being learned approximately. Requires e_sec
(secondary energy) as a new conditioning input and a steps_to_parquet.py pass
to derive it from child track first-step energies.

giant/data/loader.py

@@ -26,7 +26,9 @@ def _df_to_dict(df: pd.DataFrame) -> dict[str, np.ndarray]:
         "material": df["material"].to_numpy(dtype=object),
         "layer_id": df["layer_id"].to_numpy(dtype=np.int32),
         "n_sec": df["child_track_ids"].apply(len).to_numpy(dtype=np.int32),
+        "e_sec": df["e_sec"].to_numpy(dtype=np.float32),
         "step_length": df["step_length"].to_numpy(dtype=np.float32),
+        "post_E": df["post_E"].to_numpy(dtype=np.float32),
         "delta_e": (df["pre_E"] - df["post_E"]).to_numpy(dtype=np.float32),
         "edep": df["edep"].to_numpy(dtype=np.float32),
         "post_dir": df[["post_dx", "post_dy", "post_dz"]].to_numpy(dtype=np.float32),
@@ -63,6 +65,7 @@ _COND_COLS = [
     "material",
     "layer_id",
     "child_track_ids",
+    "e_sec",
 ]
 
 
@@ -76,6 +79,7 @@ def _cond_df_to_dict(df: pd.DataFrame) -> dict[str, np.ndarray]:
         "material": df["material"].to_numpy(dtype=object),
         "layer_id": df["layer_id"].to_numpy(dtype=np.int32),
         "n_sec": df["child_track_ids"].apply(len).to_numpy(dtype=np.int32),
+        "e_sec": df["e_sec"].to_numpy(dtype=np.float32),
     }