Add a unit test for isFromRadDecay and fix its ancestry walk

[sawenzel]

This PR adds a unit test and fixes two bugs in the isFromRadDecay query introduced in #15470.

• New test case in testStack.cxx — builds a small stack with a radioactive decay as the first secondary of its primary. Four of its checks fail on dev today.
• Primaries wrongly returned true — mTrackIDtoParticlesEntry is meaningless for a primary, since primaries never enter mParticles, so the lookup landed on an unrelated secondary.
• Descendants of a radioactive decay wrongly returned false — while (imo > 0) skipped buffer entry 0, which after FinishPrimary() is the first secondary of the current primary.
• Fix: walk trackIDs, stop at mNumberOfPrimaryParticles — the same primary test selectTracks() uses, so the mapping is never consulted for a primary.
• Const correctness, reference-vs-copy, and include fixes.

[alibuild]

Error while checking build/O2/fullCI_slc9 for bb4b31e at 2026-08-10 18:49:

## sw/BUILD/O2-full-system-test-latest/log
command /sw/slc9_x86-64/O2/slc9_x86-64-slc9_x86-64-local3/prodtests/full-system-test/dpl-workflow.sh had nonzero exit code 1
[12976:internal-dpl-injected-dummy-sink]: [18:48:29][ERROR] Failed binding socket internal-dpl-injected-dummy-sink.metric-feedback[0].push, address: ipc://@metric-feedback-0, reason: Address already in use
[12976:internal-dpl-injected-dummy-sink]: [18:48:29][ERROR] failed to attach channel metric-feedback[0] (bind on ipc://@metric-feedback-0)
[12976:internal-dpl-injected-dummy-sink]: [18:48:29][ERROR] 1 of the binding channels could not initialize. Initial configuration incomplete.
[12976:internal-dpl-injected-dummy-sink]: [18:48:29][FATAL] error while setting up workflow in o2-ctf-reader-workflow: 1 of the binding channels could not initialize. Initial configuration incomplete.
[ERROR] Workflow crashed - PID 12976 (internal-dpl-injected-dummy-sink) did not exit correctly however it's not clear why. Exit code forced to 128.
[ERROR]  - Device internal-dpl-injected-dummy-sink: pid 12976 (exit 128)
[INFO]    - First error: [18:48:29][FATAL] error while setting up workflow in o2-ctf-reader-workflow: 1 of the binding channels could not initialize. Initial configuration incomplete.
[ERROR] SEVERE: Device internal-dpl-injected-dummy-sink (12976) had at least one message above severity 7: error while setting up workflow in o2-ctf-reader-workflow: 1 of the binding channels could not initialize. Initial configuration incomplete.


## sw/BUILD/o2checkcode-latest/log
--
========== List of errors found ==========
++ GRERR=0
++ grep -v clang-diagnostic-error error-log.txt
++ grep ' error:'
grep: error-log.txt: binary file matches
++ GRERR=1
++ [[ 1 == 0 ]]
++ mkdir -p /sw/INSTALLROOT/d1bb118a6115773ad0a0b24a3cc0e461bab8cd69/slc9_x86-64/o2checkcode/1.0-local9/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='safe_int googlebenchmark bz2 ninja-fortran alibuild-recipe-tools O2-customization ninja date pytorch_cpuinfo UUID Python-modules-list MPFR CMake Alice-GRID-Utils json-c double-conversion FairCMakeModules defaults-release'
+ echo 'FULL_BUILD_REQUIRES=safe_int googlebenchmark bz2 ninja-fortran alibuild-recipe-tools O2-customization ninja date pytorch_cpuinfo UUID Python-modules-list MPFR CMake Alice-GRID-Utils json-c double-conversion FairCMakeModules defaults-release'
FULL_BUILD_REQUIRES=safe_int googlebenchmark bz2 ninja-fortran alibuild-recipe-tools O2-customization ninja date pytorch_cpuinfo UUID Python-modules-list MPFR CMake Alice-GRID-Utils json-c double-conversion FairCMakeModules defaults-release
++ 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 10
+ REVISION_VALUE=10
++ eval 'echo $LIBFFI_VERSION'
+++ echo v3.2.1-alice1
+ VERSION_VALUE=v3.2.1-alice1
[0 more errors; see full log]

Full log here.