Add run_sampling executable for sampling calorimeter geometries

Mirrors run_pbwo4 but alternates thin absorber and active layers,
with four selectable configs (by name or 1-based index): pb_scint,
fe_scint, w_scint_ecal (homogeneous ECAL-like preshower + W/scint
sampling section), and pb_lar. Each is sized to reach ~20-25 X0 of
absorber despite the dilution from inactive gaps. Wired into the
top-level CMakeLists, superbuild, and Dockerfile alongside run_pbwo4.

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

docker/Dockerfile

@@ -38,6 +38,7 @@ COPY include/ include/
 COPY bind/ bind/
 COPY lib/ lib/
 COPY run_pbwo4.cc .
+COPY run_sampling.cc .
 
 # lib/geant4/.git is a submodule pointer to the host's .git/modules dir, which
 # is not copied into the image. Replace it with a fresh repo so Geant4's cmake