Increase calorimeter size by 4x in every dimension

Scales the default transverse width (xywidth 50->200 cm) and every
layer thickness across the example/production geometries (run_pbwo4,
example.py/ipynb, run_sampling configs, export_xsec) by the same
factor, so the calo grows uniformly in x, y, and z.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

export_xsec.cc

@@ -35,7 +35,7 @@ int main(int argc, char** argv) {
 
     // Minimal valid geometry: one active PbWO4 layer so init() has something to build.
     GeometryDescriptor gd;
-    gd.addLayer(2.0, materialName, true, 1, 1);
+    gd.addLayer(8.0, materialName, true, 1, 1);
 
     G4System g4;
     g4.init(gd, -1);