[PWGCF] Femto: Allow multiple registries for SH histograms

[IgorPtak]

Problem

SH histograms scale as nCent * nKt * (2 * nJM + 2) and overflow the
512-histogram limit of a single HistogramRegistry.

Solution

PairHistManager::init() takes an optional pool of registries for SH
histograms and assigns each (cent, kT) cell to one with room. Builders
forward separate pools for same-event and mixed-event.

[github-actions]

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

[vkucera]

If you need more than 512 histograms, I think you are doing something wrong.

[alibuild]

Error while checking build/O2Physics/o2 for be6bd63 at 2026-08-05 10:23:

## sw/BUILD/O2-latest/log
CMake Error in Detectors/Base/CMakeLists.txt:

Full log here.

[alibuild]

Error while checking build/O2Physics/staging for be6bd63 at 2026-08-05 09:58:

## sw/BUILD/O2-latest/log
CMake Error in Detectors/Base/CMakeLists.txt:

Full log here.

[alibuild]

Error while checking build/O2Physics/code-check for be6bd63 at 2026-08-05 12:53:

## sw/BUILD/O2-latest/log
CMake Error in Detectors/Base/CMakeLists.txt:

Full log here.