Add next_volume/next_material columns and sampling batch script

Records the volume and material a track is about to enter when a step
ends at a geometric boundary, since post-step's physical volume still
refers to the volume the step occurred in. Also adds
run-sampling-batch.sh to run all run_sampling configs in parallel from
isolated directories, avoiding output filename collisions.

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

run-sampling-batch.sh

@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+cd "$(dirname "${BASH_SOURCE[0]}")"
+
+CONFIGS=(pb_scint fe_scint w_scint_ecal pb_lar)
+RUNS_PER_CONFIG=4
+NEVENTS=10000
+MAX_PARALLEL=16
+
+EXE="$(pwd)/build/run_sampling"
+OUTDIR="$(pwd)/sampling_batch_output"
+WORKROOT="$OUTDIR/.work"
+
+mkdir -p "$WORKROOT"
+
+run_one() {
+    local config="$1" idx="$2"
+    local workdir="$WORKROOT/${config}_${idx}"
+    mkdir -p "$workdir"
+    (
+        cd "$workdir"
+        "$EXE" "$config" "$NEVENTS"
+        mv "sampling_${config}_${NEVENTS}events_hits.root" "$OUTDIR/sampling_${config}_10k_${idx}.root"
+    )
+    rmdir "$workdir"
+}
+export -f run_one
+export EXE NEVENTS OUTDIR WORKROOT
+
+jobs=()
+for config in "${CONFIGS[@]}"; do
+    for idx in $(seq 0 $((RUNS_PER_CONFIG - 1))); do
+        jobs+=("$config $idx")
+    done
+done
+
+printf '%s\n' "${jobs[@]}" | xargs -P "$MAX_PARALLEL" -L 1 bash -c 'run_one "$1" "$2"' _
+
+rmdir "$WORKROOT"