Update trigger for use with a trigger plane and ensure backscatter is excluded

As spotted by @anatolii.korol, there are (rare) edge cases close to a geometrical region excluded from fast sim where secondaries created in a full sim shower can cross the trigger boundary and cause unwanted fast sim triggers inside the calorimeter. This MR proposes a fix for this with two important steps:

1. Adding a trigger plane to the compact file right at the front of the calorimeter face (and of course adjusting the model.RegionName in scripts/ddsim_steer.py ).

   E.g. for the ECAL Barrel of ILD_l5_o1_v02.xml this would mean creating a local copy of k4geo and modifying SEcal06_hybrid_Barrel.xml accordingly:

   diff --git a/ILD/compact/ILD_common_v02/SEcal06_hybrid_Barrel.xml b/ILD/compact/ILD_common_v02/SEcal06_hybrid_Barrel.xml
   index 457246d..d66595f 100644
   --- a/ILD/compact/ILD_common_v02/SEcal06_hybrid_Barrel.xml
   +++ b/ILD/compact/ILD_common_v02/SEcal06_hybrid_Barrel.xml
   @@ -5,8 +5,35 @@
        </region>
      </regions>
    
   +  <regions>
   +    <region name="EcalBarrelFastSimTriggerRegion">
   +    </region>
   +  </regions>
   +
   +  <readouts>
   +        <readout name="ECalBarrelFaceReadout">
   +            <id>system:5,side:2,layer:9,module:8,sensor:8</id>
   +        </readout>
   +    </readouts>
   +
      <detectors>
    
   +    <detector name="EcalBarrelFastSimTrigger" type="PolyhedralBarrelSurfaces" region="EcalBarrelFastSimTriggerRegion" readout="ECalBarrelFaceReadout">
   +      <comment> EM Calorimeter Fast Sim Trigger Region </comment>
   +       
   +      <envelope vis="ILD_ECALVis">
   +        <shape type="PolyhedraRegular" numsides="Ecal_symmetry"  rmin="Ecal_inner_radius - 3*env_safety" 
   +        rmax="Ecal_inner_radius - 2*env_safety" dz="2.*Ecal_half_length + env_safety"  material = "Air" />
   +        <rotation x="0*deg" y="0*deg" z="-180*deg/Ecal_symmetry"/>
   +      </envelope>
   +      
   +      <dimensions numsides="Ecal_symmetry" rmin="Ecal_inner_radius - 3*env_safety" 
   +      rmax="Ecal_inner_radius - 2*env_safety" zhalf="Ecal_half_length"
   +      phi0="-180*deg/Ecal_symmetry" material = "Air" />
   +
   +  </detector>
   +
   +
        <detector name="EcalBarrel" type="SEcal06_Barrel" id="ILDDetID_ECAL" readout="EcalBarrelCollection" vis="ILD_BlueVis"
         region="EcalBarrelRegion">
   

2. Ensure that backscatter from the calorimeter does not cause model triggers (see changes to src/OctogonalBarrelTrigger.cc)

Validation checks performed so far (with 2025-01-28 key4hep release):

• particle gun into calo in FastSim region

  position: (0, 0, 0), direction: (0 1 0), particle: gamma, energy: 50 GeV

  Results: Triggers Fast Sim (expected behavior)

• particle gun inside calo (firing backwards) in FastSim region

  position: (0, 1860, 0), direction: 0 -1 0, particle: gamma, energy: 50 GeV

  Results: Does not trigger Fast Sim (expected behavior)

• particle gun into calo region excluded from FastSim (corner of barrel)

  position: (0, 0, 0), direction: (-0.4152 1 0), particle: gamma, energy: 50 GeV

  Results: Does not trigger Fast Sim (expected behavior)

@anatolii.korol would you be able to quickly check if this resolves the issue you were seeing?