FT3 Staves Minor ambiguity bugfix

[JustusRudolph]

• This PR makes naming for the active and inactive parts of the sensor unambiguous. Earlier there was no way to tell Geant to only look at the active part.

• Additionally, by removing one count incrementer, the inactive and active parts of the same sensor are now accurately tagged together, making indexing of sensors more accurate and future proof.

[alibuild]

Error while checking build/O2/fullCI_slc9 for da1c7be at 2026-06-10 20:37:

## sw/BUILD/O2Physics-latest/log
CMake Error at /sw/slc9_x86-64/CMake/v4.1.4-2/share/cmake-4.1/Modules/CMakeTestCCompiler.cmake:67 (message):
    ninja: build stopped: subcommand failed.

Full log here.