[PWGLF] Add histograms for antineutron closure test

[FabiolaLP]

• Added antineutron transverse-momentum histograms for two independent closure samples.
• Added histograms to evaluate the antineutron transverse-momentum distribution after the ITS selection.
• Added momentum-response histograms for the CEX pair at MC and reconstructed levels.

These histograms are intended for closure tests and validation of the antineutron momentum reconstruction and selection efficiency.

[github-actions]

O2 linter results: ❌ 1 errors, ⚠️ 18 warnings, 🔕 0 disabled

[alibuild]

Error while checking build/O2Physics/code-check for 10c00ad at 2026-09-06 16:50:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 1 errors and 56 warnings.'
Found 1 errors and 56 warnings.
++ [[ 1 -gt 0 ]]
++ cat /sw/BUILD/cd24971b439f73fe43e10fcf8d84cd5018d06004/O2Physics-code-check/errors.txt
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:223:35: error: do not implicitly decay an array into a pointer; consider using gsl::array_view or an explicit cast instead [cppcoreguidelines-pro-bounds-array-to-pointer-decay,-warnings-as-errors]
++ [[ 56 -gt 0 ]]
++ cat /sw/BUILD/cd24971b439f73fe43e10fcf8d84cd5018d06004/O2Physics-code-check/warnings.txt
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:222:5: warning: do not declare C-style arrays, use 'std::array' instead [modernize-avoid-c-arrays]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:283:92: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:293:92: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:303:92: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:310:105: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:334:36: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:352:78: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:354:82: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:362:50: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:364:110: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:373:36: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:387:50: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:391:109: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:400:36: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:411:35: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:417:50: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:421:106: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:430:36: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:457:50: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:461:109: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:470:34: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:491:31: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:494:30: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:560:84: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:563:30: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:565:31: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:567:31: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:570:33: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:572:34: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:575:33: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:577:34: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:591:84: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:635:41: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:638:45: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:678:34: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:709:34: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:723:45: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:731:38: warning: statement should be inside braces [readability-braces-around-statements]
PWGLF/TableProducer/Nuspex/nucleiAntineutronCex.cxx:733:38: warning: statement should be inside braces [readability-braces-around-statements]
[0 more errors; see full log]

Full log here.