Set the number of Geant4 threads before TGeant4 is created

This sizes TGeo for the requested number of transport threads.

- TGeant4's constructor sets gGeoManager->SetMaxThreads from Geant4's thread count, before /run/numberOfThreads ran.
- The count is now passed in G4FORCENUMBEROFTHREADS before TGeant4 is created.

https://its.cern.ch/jira/browse/O2-7305

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: sawenzel

Detectors/gconfig/g4Config.C

@@ -167,6 +167,11 @@ void Config()
   }
   /// avoid the use of G4BACKTRACE (it seems to inferfere with process logic in o2-sim)
   setenv("G4BACKTRACE", "none", 1);
+  if (mtMode) {
+    // one transport thread per requested worker; set before TGeant4 exists, since its constructor sizes TGeo for the threads
+    const char* nthreads = getenv("ALICE_NSIMWORKERS");
+    setenv("G4FORCENUMBEROFTHREADS", nthreads ? nthreads : "1", 1);
+  }
   /// Create the G4 VMC
   TGeant4* geant4 = new TGeant4("TGeant4", "The Geant4 Monte Carlo", runConfiguration);
   std::cout << "Geant4 has been created." << std::endl;
@@ -184,9 +189,7 @@ void Config()
     if (g4Params.navmode == o2::conf::EG4Nav::kVecGeom) {
       LOG(fatal) << "G4.navmode=kVecGeom is not yet supported in multithreaded mode (--isMT)";
     }
-    // one transport thread per requested worker, each taking one sub-event at a time
-    const char* nthreads = getenv("ALICE_NSIMWORKERS");
-    geant4->ProcessGeantCommand(Form("/run/numberOfThreads %s", nthreads ? nthreads : "1"));
+    // each thread takes one sub-event at a time
     geant4->ProcessGeantCommand("/run/eventModulo 1");
     // a thread without more work ends its own event loop after the current (empty) event
     o2::base::MTWorkerHooks::instance().stopWorker = []() { G4RunManager::GetRunManager()->AbortRun(true); };
@@ -196,8 +199,7 @@ void Config()
     o2::base::MTWorkerHooks::instance().beginTrack = &o2::simsetup::invalidateFieldCache;
     // the primary generator exists only on the worker threads, so its commands are applied there
     o2::base::MTWorkerHooks::instance().beginRunOnWorker = []() { G4UImanager::GetUIpointer()->ApplyCommand(kSkipUnknownParticles); };
-    // the media energy cuts are set after the initialisation run has built all couples, so Geant4 would not apply them;
-    // marking the region cuts as modified makes the first run recompute the couples and rebuild the physics tables
+    // the media energy cuts come after the initialisation run built the couples, so the first run must rebuild them
     o2::base::MTWorkerHooks::instance().beforeTransport = []() {
       for (auto region : *G4RegionStore::GetInstance()) {
         if (auto cuts = region->GetProductionCuts()) {