TPC track reader: do not accumulate MC labels over entries

With one entry per call, TrackReader::accumulate swapped the tracks and
cluster references of the new entry in, but appended its MC labels to
mMCTruthOut, which is never cleared. From the second tree entry on, the
consumers therefore got the labels of the first entry for the tracks of
the current one (label i = label of track i of entry 0). Swap the labels
as well.

Only visible when a tpctracks.root holds more than one TF (e.g. merged
MC files); O2DPG writes one TF per file.

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

Author: 2 people

Detectors/TPC/workflow/readers/src/TrackReaderSpec.cxx

@@ -97,7 +97,7 @@ void TrackReader::accumulate(int from, int n)
     mTracksOut.swap(*mTracksInp);
     mCluRefVecOut.swap(*mCluRefVecInp);
     if (mUseMC) {
-      std::copy(mMCTruthInp->begin(), mMCTruthInp->end(), std::back_inserter(mMCTruthOut));
+      mMCTruthOut.swap(*mMCTruthInp);
     }
   } else {
     for (int iev = 0; iev < n; iev++) {