Keep strange tracks in decay order also with one vertexer thread

This makes the order of the tracked strangeness tables within a collision the same in MC as in data.

- SVertexer sorted the strange tracks by decay only when running with more than one thread; MC reconstruction runs it with one.
- The strange tracks are now sorted by decay in all cases.
- The AOD producer groups them by collision with a stable sort, so the decay order within a collision is kept.

https://its.cern.ch/jira/browse/O2-7197

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: sawenzel

Detectors/AOD/src/AODProducerWorkflowSpec.cxx

@@ -1527,7 +1527,7 @@ void AODProducerWorkflowDPL::prepareStrangenessTracking(const o2::globaltracking
   std::exclusive_scan(mVertexStrLUT.begin(), mVertexStrLUT.end(), mVertexStrLUT.begin(), 0);
 
   // sort by collision ID
-  std::sort(mCollisionStrTrk.begin(), mCollisionStrTrk.end(), [](const auto& a, const auto& b) { return a.first < b.first; });
+  std::stable_sort(mCollisionStrTrk.begin(), mCollisionStrTrk.end(), [](const auto& a, const auto& b) { return a.first < b.first; });
   mStrTrkIndices.clear();
   mStrTrkIndices.resize(mCollisionStrTrk.size(), -1);
 }

Detectors/Vertexing/src/SVertexer.cxx

@@ -225,8 +225,8 @@ void SVertexer::produceOutput(o2::framework::ProcessingContext& pc)
 
     std::vector<int> sortIdx(strTracksTmp.size());
     std::iota(sortIdx.begin(), sortIdx.end(), 0);
-    // if mNTreads > 1 we need to sort tracks, clus and MCLabs by their mDecayRef
-    if (mNThreads > 1 && mNStrangeTracks > 1) {
+    // sort tracks, clus and MCLabs by their mDecayRef, also with one thread, so that they follow the vertex order
+    if (mNStrangeTracks > 1) {
       std::sort(sortIdx.begin(), sortIdx.end(), [&strTracksTmp](int i1, int i2) { return strTracksTmp[i1].mDecayRef < strTracksTmp[i2].mDecayRef; });
     }