[mbarlou]

Summary

This PR adds event-level forward and backward multiplicity QA as a
function of centrality and the forward-backward pseudorapidity
separation gap to stronglyIntensiveCorr.

Two three-dimensional histograms are introduced:

• QA/fbAllGaps/nFvsCentGap
• QA/fbAllGaps/nBvsCentGap

Their axes are centrality, DeltaEta, and forward/backward
multiplicity.

For cfgEtaWidth = 0.2, the histograms store the seven
non-overlapping physical gaps:

DeltaEta = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2.

Motivation

The existing task provides event-level multiplicity QA only for the
default forward-backward windows. The additional histograms are
needed to inspect the multiplicity distributions and sparse outlier
populations independently for every separation gap.

Scope

The modification adds QA output only. It does not change:

• event selection;
• track selection;
• centrality selection;
• existing moment profiles;
• subsample assignment;
• derived physics observables.

Validation

• O2Physics builds successfully.
• A local Pb-Pb Run 3 test is in progress.
• The output will be checked for both new TH3 histograms.
• The DeltaEta-axis centres will be verified to be:
  0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2.

[github-actions]

O2 linter results: ❌ 0 errors, ⚠️ 1 warnings, 🔕 0 disabled

[mbarlou]

Local validation completed successfully on Pb-Pb Run 3 data. The workflow finished with status 0. Both new TH3 histograms, QA/fbAllGaps/nFvsCentGap and QA/fbAllGaps/nBvsCentGap, are present. Their DeltaEta-axis bin centres are 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 and 1.2, and the forward and backward histograms have matching entry counts.