[afroch]

Summary

Adds JetGhostLabels::associate_ghosts, which reruns the jet clustering with a set of truth particles added at a negligible momentum (1e-18 of their own) and returns for each of them the index of the jet it ended up in. The ghosts are matched back to the jets the analysis already built through their shared constituents, so the result does not depend on the jet ordering and works with any JetClustering::clustering_* functor, inclusive or exclusive. A ghost that ends up outside every jet gets -1.

ExclusiveJetClusteringHelper now exposes its clustering call (clustering_spec), its input pseudojets and the constituent index column, so the same clustering can be rerun from Python. The clustering itself is unchanged.

This is the first of six PRs adding ATLAS-style truth information for flavour tagging studies: ghost-associated hadron and parton labels, truth labels for jet constituents and matched truth jets. The series builds on the truth labelling that @vincentriechers developed on his truth-labelling branch.

Testing

• Unit tests check that ghosts leave the jet momenta unchanged and that ghosts along two separate prongs end up in different jets.
• In a standalone check with the same ee_kt clustering on 2000 wzp6 H→ss and 1000 Pythia8 Z→bb events, every b and c hadron ghost ended up in a jet and no jet axis moved.
• ctest: 54/54.