add superbuild to compile Geant4 from submodule source

superbuild/CMakeLists.txt uses ExternalProject_Add to give three targets:

  cmake -B build -S superbuild/
  cmake --build build --target geant4      # build Geant4 → build/geant4-install
  cmake --build build --target run_pbwo4   # build minicalosim against it
  cmake --build build                      # both

The existing root CMakeLists.txt (find_package Geant4) is unchanged
for the system-Geant4 workflow.

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

superbuild/CMakeLists.txt

@@ -0,0 +1,48 @@
+cmake_minimum_required(VERSION 3.16)
+project(minicalosim-superbuild NONE)
+
+include(ExternalProject)
+include(ProcessorCount)
+
+ProcessorCount(NPROC)
+if(NOT NPROC OR NPROC EQUAL 0)
+  set(NPROC 4)
+endif()
+# Cap at 16 to avoid memory exhaustion on large machines (mirrors Dockerfile)
+if(NPROC GREATER 16)
+  set(NPROC 16)
+endif()
+
+set(G4_INSTALL ${CMAKE_BINARY_DIR}/geant4-install)
+set(G4_SRC     ${CMAKE_SOURCE_DIR}/../lib/geant4)
+set(MINI_SRC   ${CMAKE_SOURCE_DIR}/..)
+
+option(GEANT4_INSTALL_DATA "Download Geant4 physics data files (~2 GB)" ON)
+
+# ── Geant4 ────────────────────────────────────────────────────────────────────
+ExternalProject_Add(geant4
+  SOURCE_DIR  ${G4_SRC}
+  BINARY_DIR  ${CMAKE_BINARY_DIR}/geant4-build
+  INSTALL_DIR ${G4_INSTALL}
+  CMAKE_ARGS
+    -DCMAKE_INSTALL_PREFIX=<INSTALL_DIR>
+    -DCMAKE_BUILD_TYPE=Release
+    -DGEANT4_INSTALL_DATA=${GEANT4_INSTALL_DATA}
+    -DGEANT4_USE_GDML=ON
+    -DGEANT4_BUILD_MULTITHREADED=OFF
+    -DGEANT4_BUILD_TLS_MODEL=global-dynamic
+  BUILD_COMMAND   cmake --build <BINARY_DIR> --parallel ${NPROC}
+  INSTALL_COMMAND cmake --install <BINARY_DIR>
+)
+
+# ── minicalosim (run_pbwo4) ───────────────────────────────────────────────────
+ExternalProject_Add(run_pbwo4
+  SOURCE_DIR  ${MINI_SRC}
+  BINARY_DIR  ${CMAKE_BINARY_DIR}/minicalosim-build
+  CMAKE_ARGS
+    -DCMAKE_PREFIX_PATH=${G4_INSTALL}
+    -DWITH_GEANT4_UIVIS=OFF
+  BUILD_COMMAND   cmake --build <BINARY_DIR> --target run_pbwo4 --parallel ${NPROC}
+  INSTALL_COMMAND ""
+  DEPENDS geant4
+)