ITS: cell neighbour step use atomicMax

[f3sch]

Brings to the logic to calculate the cell levels close to what is done in GPU and also makes it clearer what is happening.

[github-actions]

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[alibuild]

Error while checking build/O2/fullCI_slc9 for f6f9e1f at 2025-08-20 11:11:

## sw/BUILD/O2-full-system-test-latest/log
Detected critical problem in logfile digi.log
digi.log:[20225:internal-dpl-ccdb-backend]: [11:11:11][ERROR] Exception while running: Fatal error. Rethrowing.
digi.log-[20225:internal-dpl-ccdb-backend]: [11:11:11][FATAL] Unhandled o2::framework::runtime_error reached the top of main of o2-sim-digitizer-workflow, device shutting down. Reason: Fatal error
[20225:internal-dpl-ccdb-backend]: [11:11:09][ERROR] CCDBDownloader CURL transfer error - Timeout was reached
[20225:internal-dpl-ccdb-backend]: [11:11:09][ERROR] CcdbDownloader finished transfer http://alice-ccdb.cern.ch/CTP/Calib/OrbitReset for 1550600800000 (agent_id: alimetal03.cern.ch-1755681058-7bJbTt) with http code: 0
[20225:internal-dpl-ccdb-backend]: [11:11:09][ERROR] File CTP/Calib/OrbitReset could not be retrieved. No more hosts to try.
[20225:internal-dpl-ccdb-backend]: [11:11:09][FATAL] Unable to find CCDB object CTP/Calib/OrbitReset/1550600800000
[20225:internal-dpl-ccdb-backend]: [11:11:11][ERROR] Exception while running: Fatal error. Rethrowing.
[20225:internal-dpl-ccdb-backend]: [11:11:11][FATAL] Unhandled o2::framework::runtime_error reached the top of main of o2-sim-digitizer-workflow, device shutting down. Reason: Fatal error
[ERROR] Workflow crashed - PID 20225 (internal-dpl-ccdb-backend) did not exit correctly however it's not clear why. Exit code forced to 128.
[ERROR]  - Device internal-dpl-ccdb-backend: pid 20225 (exit 128)
[INFO]    - First error: [11:11:09][FATAL] Unable to find CCDB object CTP/Calib/OrbitReset/1550600800000
[ERROR] SEVERE: Device internal-dpl-ccdb-backend (20225) had at least one message above severity 7: Unable to find CCDB object CTP/Calib/OrbitReset/1550600800000


## sw/BUILD/o2checkcode-latest/log
--
========== List of errors found ==========
++ GRERR=0
++ grep -v clang-diagnostic-error error-log.txt
++ grep ' error:'
++ GRERR=1
++ [[ 1 == 0 ]]
++ mkdir -p /sw/INSTALLROOT/3156ea1d52c1c034d419f0969128d8f3b4665160/slc9_x86-64/o2checkcode/1.0-local682/etc/modulefiles
++ cat
--

Full log here.