extend Steps ntuple and switch Docker to Geant4 submodule source

Add 15 new columns to the Steps ntuple: edep, step_length, parent_id,
process name, layer_id, material, pre-step momentum direction, and
B/E field components (in T and V/m) at the pre-step point.

Replace wget Geant4 tarball download in all three Dockerfiles with
ADD of the lib/geant4 submodule source; narrow submodule init to
lib/pybind11 only so builds no longer require network access for Geant4.

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

docker/Dockerfile-mini

@@ -16,23 +16,21 @@ RUN python3 -m pip install --upgrade pip && \
     python3 -m pip install numpy pandas uproot awkward plotly ipython && \
     ln -sf /usr/bin/python3 /usr/bin/python
 
-# Geant4 v11.1.2 — batch mode only (no visualization, no multithreading)
-RUN wget -q https://gitlab.cern.ch/geant4/geant4/-/archive/v11.1.2/geant4-v11.1.2.tar.gz && \
-    tar -xzf geant4-v11.1.2.tar.gz -C /tmp && \
-    rm geant4-v11.1.2.tar.gz && \
-    mkdir /tmp/geant4-v11.1.2-build && \
-    cmake \
-        -S /tmp/geant4-v11.1.2 \
-        -B /tmp/geant4-v11.1.2-build \
+# Geant4 — build from submodule source (batch mode only, no visualization, no multithreading)
+ADD minicalosim/lib/geant4 /tmp/geant4-src
+
+RUN cmake \
+        -S /tmp/geant4-src \
+        -B /tmp/geant4-build \
         -DCMAKE_INSTALL_PREFIX=/opt/geant4-v11.1.2 \
         -DCMAKE_RULE_MESSAGES=OFF \
         -DGEANT4_INSTALL_DATA=ON \
         -DGEANT4_BUILD_MULTITHREADED=OFF \
         -DGEANT4_BUILD_TLS_MODEL=global-dynamic \
         > /dev/null && \
-    cmake --build /tmp/geant4-v11.1.2-build --parallel $(nproc) > /dev/null && \
-    cmake --install /tmp/geant4-v11.1.2-build > /dev/null && \
-    rm -rf /tmp/geant4-v11.1.2 /tmp/geant4-v11.1.2-build
+    cmake --build /tmp/geant4-build --parallel $(nproc) > /dev/null && \
+    cmake --install /tmp/geant4-build > /dev/null && \
+    rm -rf /tmp/geant4-src /tmp/geant4-build
 
 ENV LD_LIBRARY_PATH="/opt/geant4-v11.1.2/lib"
 
@@ -45,7 +43,7 @@ LABEL org.label-schema.build-date=$BUILD_DATE
 ADD minicalosim /root/minicalosim
 
 RUN cd /root/minicalosim && git checkout $COMMIT && \
-    git submodule update --init && \
+    git submodule update --init lib/pybind11 && \
     cmake \
         -S . -B build \
         -DWITH_GEANT4_UIVIS=OFF \