[PWGEM] add new pairQC Task + clean up photonhbt

[Steffimro]

New task pairQcTask: analysis of pairs of V0 photon candidates (same event only). The following analyses are possible:

• pair observables before and after the pair cuts: conversion-point distances, opening angle, (dEta, dPhi), leg classes, pointing to the primary vertex
• duplicate-candidate QA: leg-level similarity of candidates of one collision, with the MC classification of duplicate pairs
• cross-pair QA: the two alternative e+e- pairings of the four legs, built with the DCAFitter and with the analytic helix builder
  (Utils/AnalyticV0.h), and the veto derived from them
• MC: pair truth type, fake subtype, ancestry of the parent photons, true conversion points and q_true vs q_reco

added new functions into MCUtilities and PairUtilities, so that multiple tasks can use those functions to analyse certain reconstruction artifacts.

Moved the PairQA out from photonhbt, so that only the correlation function analysis and the MC analysis stays there. Added a new Mixing with V0 Legs only, based on V0Leg information and V0LegXYZ information.

[github-actions]

O2 linter results: ❌ 0 errors, ⚠️ 52 warnings, 🔕 114 disabled

[mhemmer-cern]

Please check the CMakeList changes again and while you are at it take a look at the code-check results to at least fix the introduced errors please.

[mhemmer-cern]

ranges is actually used in this header file for std::ranges::input_range. Could you add it back in please

[alibuild]

Error while checking build/O2Physics/staging for f7f1685 at 2026-09-20 21:39:

No log files found

Full log here.

[mhemmer-cern]

Looks good to me now

[mhemmer-cern]

Since code check errors found here were fixed I will enable auto-merge, so this can make it into tomorrows tag without needing to wait for the builds

[alibuild]

Error while checking build/O2Physics/code-check for f7f1685 at 2026-09-20 21:53:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 5 errors and 1 warnings.'
Found 5 errors and 1 warnings.
++ [[ 5 -gt 0 ]]
++ cat /sw/BUILD/f48fa235c7a7a194409671a71a421fd2ba3a36bb/O2Physics-code-check/errors.txt
PWGEM/PhotonMeson/Utils/AnalyticV0.h:25:1: error: included header Cartesian.h is not used directly [misc-include-cleaner,-warnings-as-errors]
PWGEM/PhotonMeson/Utils/AnalyticV0.h:161:27: error: no header providing "o2::math_utils::Point3D" is directly included [misc-include-cleaner,-warnings-as-errors]
PWGEM/PhotonMeson/Utils/MCUtilities.h:104:72: error: no header providing "std::ranges::input_range" is directly included [misc-include-cleaner,-warnings-as-errors]
PWGEM/PhotonMeson/Utils/PairUtilities.h:24:1: error: included header Vector3D.h is not used directly [misc-include-cleaner,-warnings-as-errors]
PWGEM/PhotonMeson/Utils/PairUtilities.h:26:1: error: included header Vector4D.h is not used directly [misc-include-cleaner,-warnings-as-errors]
++ [[ 1 -gt 0 ]]
++ cat /sw/BUILD/f48fa235c7a7a194409671a71a421fd2ba3a36bb/O2Physics-code-check/warnings.txt
PWGEM/PhotonMeson/Tasks/pairQcTask.cxx:1007:19: warning: use auto when initializing with a cast to avoid duplicating the type name [modernize-use-auto]
++ [[ 5 -gt 0 ]]
++ exit 1
--

Full log here.

[alibuild]

Error while checking build/O2Physics/code-check for 2d9c546 at 2026-09-20 22:05:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 4 errors and 1 warnings.'
Found 4 errors and 1 warnings.
++ [[ 4 -gt 0 ]]
++ cat /sw/BUILD/e3eff75f905d9fa2d4ba76d28d779a5e821d7ed4/O2Physics-code-check/errors.txt
PWGEM/PhotonMeson/Utils/AnalyticV0.h:25:1: error: included header Cartesian.h is not used directly [misc-include-cleaner,-warnings-as-errors]
PWGEM/PhotonMeson/Utils/AnalyticV0.h:161:27: error: no header providing "o2::math_utils::Point3D" is directly included [misc-include-cleaner,-warnings-as-errors]
PWGEM/PhotonMeson/Utils/PairUtilities.h:24:1: error: included header Vector3D.h is not used directly [misc-include-cleaner,-warnings-as-errors]
PWGEM/PhotonMeson/Utils/PairUtilities.h:26:1: error: included header Vector4D.h is not used directly [misc-include-cleaner,-warnings-as-errors]
++ [[ 1 -gt 0 ]]
++ cat /sw/BUILD/e3eff75f905d9fa2d4ba76d28d779a5e821d7ed4/O2Physics-code-check/warnings.txt
PWGEM/PhotonMeson/Tasks/pairQcTask.cxx:1007:19: warning: use auto when initializing with a cast to avoid duplicating the type name [modernize-use-auto]
++ [[ 4 -gt 0 ]]
++ exit 1
--

Full log here.