Restrict the TRD PAI model to 14 particle species

Reduce the Geant4 initialisation time by setting up the PAI model only for the charged species that matter in the TRD gas, instead of all 99. This reduces the PAI setup from 0.81 s to 0.11 s per o2-sim worker.

|                                             | all (99)          | 14 species |
|---------------------------------------------|-------------------|------------|
| PAI model instances                         | 99                | 14         |
| PAI setup time                              | 809 ms            | 113 ms     |
| boxgen, 1 event: hits / steps               | 14986 / 17871     | identical  |
| 5 pythia8pp events: hits / steps            | 456275 / 530601   | identical  |
| 5 pythia8pp events: TRD hits / total charge | 69847 / 2.42322e6 | identical  |

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

Author: sawenzel

Detectors/gconfig/g4config.in

@@ -32,7 +32,7 @@
 # Geant4 VMC >= v3.2
 /mcPhysics/emModel/setEmModel  PAI
 /mcPhysics/emModel/setRegions  TRD_Gas-mix
-/mcPhysics/emModel/setParticles  all
+/mcPhysics/emModel/setParticles  e- e+ mu- mu+ pi- pi+ kaon- kaon+ proton anti_proton deuteron triton He3 alpha
 /mcPrimaryGenerator/skipUnknownParticles true # don't crash when seeing unknown ion etc. (issue warning)
 
 #