[PWGEM] Unify QC + MaterialBudgetTasks

[Steffimro]

This task restructures the PCM QC code: the standalone pcmQCMC task and the material-budget part of MaterialBudget.cxx are merged into pcm-qc as process functions, the old tasks are moved to the legacy folder.

Process functions (all off by default except processQC):

processQC / processQCML: data QC as before, with classical or ML-based selection. Added default histograms: Rxy/eta/phi vs pT, an (alpha, qT, pT) sparse for pT-sliced Armenteros plots, psi_pair vs pT (ML path), TPC nsigma_el vs eta.

processPCMQCMC / processPCMQCMCML: former pcmQCMC. QC observables split by MC truth class (primary, fromWD, fromHS, fromPi0Dalitz, fromEtaDalitz, candidate as purity denominator) including pT/eta/phi and conversion-point resolution. MC histograms are only booked when an MC process is enabled.

processGen: generator-level reference (needs binned generated-pT derived data), plus wide-R conversion-point maps

processRecoQA: new, runs on AO2Ds before skimming. Single-photon efficiency analysis possible (converted -> both legs tracked -> both legs in V0Legs -> candidate in V0PhotonsKF) as {pT, eta, phi, Rconv} sparses, duplicate counters and missing-leg diagnostics, to separate tracking losses from SVertexer losses.

New optional groups: materialBudgetSettings_group (conversion-point maps, overall and per detector region), mcAnalysisModeSettings_group (detailed resolution, collision-association QA, true-association requirement), genSettings_group, recoQASettings_group, qaSettings_group (cut variables vs pT before/after the selection). All new output is off by default.

[github-actions]

O2 linter results: ❌ 6 errors, ⚠️ 25 warnings, 🔕 23 disabled

[alibuild]

Error while checking build/O2Physics/code-check for 1583228 at 2026-09-02 18:20:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 1 errors and 0 warnings.'
Found 1 errors and 0 warnings.
++ [[ 1 -gt 0 ]]
++ cat /sw/BUILD/5ea47ac9465c0fd9a67b00eabc4353bd34a8775a/O2Physics-code-check/errors.txt
PWGEM/PhotonMeson/Tasks/pcmQC.cxx:76:1: error: included header unordered_set is not used directly [misc-include-cleaner,-warnings-as-errors]
++ [[ 0 -gt 0 ]]
++ [[ 1 -gt 0 ]]
++ exit 1
--

Full log here.

[mhemmer-cern]

It looks like the old tasks are not in the Legacy folder with this commit.