[sawenzel]

This PR adds an optional TPC digitizer device that digitizes all sectors in one process with several threads, instead of one process per lane.

With lanes, every process loads and recalculates its own copy of the space-charge distortion maps, which in PbPb costs about 1 GB of memory and several minutes of CPU per lane. In the new device the maps are prepared once and shared by all threads, while each thread owns a digitizer with its own random rings, hit chains and output file. It is enabled with --tpc-digi-threads N together with --tpc-chunked-writer; the chunked merger and everything downstream are unchanged.

The effect on one anchored PbPb timeframe (run 572106, distortion type 2) is substantial:

	8 lanes	8 threads
peak PSS	15.05 GiB	7.95 GiB
wall time	216 s	69 s

The lanes run with --n-threads-distortions 1, as in O2DPG, and the threaded device with 8.

Note that nothing changes by default. The digits of the threaded device are identical to those of the lane spec with --tpc-sector-seeding for 1, 4 and 8 lanes and threads, with and without distortions, also when run through the O2DPG workflow up to the TPC clusterization.

This builds on #16082, #16083 and #16085, which are merged. It moves the per-sector loop into a shared function, prepares the digitizer for concurrent use and adds the device itself. The last two commits address review comments.

Overall, this halves the memory of the TPC digitization and makes it about three times faster.

https://its.cern.ch/jira/browse/O2-7309

Assisted by Claude Code.

[sawenzel]

Added commits after internal review.

[alibuild]

Error while checking build/O2/fullCI_slc9 for 57422d7 at 2026-11-11 05:18:

## sw/BUILD/O2-sim-challenge-test-latest/log
./sim-challenge.log[ERROR] SHUTTING DOWN DUE TO SIGNALED EXIT IN COMPONENT 208
./sim.log[ERROR] SHUTTING DOWN DUE TO SIGNALED EXIT IN COMPONENT 208


## sw/BUILD/o2checkcode-latest/log
--
========== List of errors found ==========
++ echo 'Found 0 errors and 0 warnings.'
Found 0 errors and 0 warnings.
++ [[ 0 -gt 0 ]]
++ [[ 0 -gt 0 ]]
++ [[ 0 -gt 0 ]]
++ mkdir -p /sw/INSTALLROOT/0f36dfe2fa6d50f46c47eb8336eeef00082e4a49/slc9_x86-64/o2checkcode/1.0-local64/etc/modulefiles
++ alibuild-generate-module
+ HAS_BIN=
+ HAS_LIB=
+ HAS_CMAKE=
+ HAS_ROOT=
+ '[' 0 '!=' 0 ']'
+ cat
+ printf 'if ![ is-loaded '\''BASE/1.0'\'' ] {\n module load BASE/1.0\n}'
+ echo 'BUILD_REQUIRES=CMake alibuild-recipe-tools defaults-release'
BUILD_REQUIRES=CMake alibuild-recipe-tools defaults-release
+ FULL_BUILD_REQUIRES='Python-modules-list MPFR googlebenchmark UUID date defaults-release ninja-fortran bz2 pytorch_cpuinfo json-c Alice-GRID-Utils ninja alibuild-recipe-tools CMake FairCMakeModules O2-customization double-conversion safe_int'
+ echo 'FULL_BUILD_REQUIRES=Python-modules-list MPFR googlebenchmark UUID date defaults-release ninja-fortran bz2 pytorch_cpuinfo json-c Alice-GRID-Utils ninja alibuild-recipe-tools CMake FairCMakeModules O2-customization double-conversion safe_int'
FULL_BUILD_REQUIRES=Python-modules-list MPFR googlebenchmark UUID date defaults-release ninja-fortran bz2 pytorch_cpuinfo json-c Alice-GRID-Utils ninja alibuild-recipe-tools CMake FairCMakeModules O2-customization double-conversion safe_int
++ env
++ cut -f1 -d=
++ grep -v '^DEFAULT_'
++ grep -v PKGREVISION
++ grep -v ALIBUILD_RECIPE_TOOLS
++ grep REVISION
++ sed -e s/_REVISION//
+ for x in $(env | cut -f1 -d= | grep -v "^DEFAULT_" | grep -v PKGREVISION | grep -v ALIBUILD_RECIPE_TOOLS | grep REVISION | sed -e 's/_REVISION//')
++ eval 'echo $LIBFFI_REVISION'
+++ echo 14
+ REVISION_VALUE=14
++ eval 'echo $LIBFFI_VERSION'
+++ echo v3.2.1-alice1
+ VERSION_VALUE=v3.2.1-alice1
++ eval 'echo $LIBFFI_ROOT'
+++ echo /sw/slc9_x86-64/libffi/v3.2.1-alice1-14
+ ROOT_PATH_VALUE=/sw/slc9_x86-64/libffi/v3.2.1-alice1-14
+ '[' -z 14 ']'
+ echo 'Python-modules-list MPFR googlebenchmark UUID date defaults-release ninja-fortran bz2 pytorch_cpuinfo json-c Alice-GRID-Utils ninja alibuild-recipe-tools CMake FairCMakeModules O2-customization double-conversion safe_int'
+ tr '[:lower:]' '[:upper:]'
+ tr - _
+ tr ' ' '\n'
[0 more errors; see full log]

Full log here.