[PWGLF] eventShapeCoex: Add forward/backward sub-event producer

[c-andrei]

This is the first commit of this task. It starts the migration to O2Physics of the event-shape analyses developed in AliPhysics under PWGLF/SPECTRA/MultEvShape.
Adds a table producer (o2-analysis-lf-event-shape-coex) that writes per-collision forward/backward sub-event quantities at reconstructed and generator level, plus an MC label table linking the two. It is the first GEP table producer, so it creates PWGLF/TableProducer/GlobalEventProperties (already listed in CODEOWNERS). "Coex" in the names is the internal name of the analysis.

[github-actions]

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

[alibuild]

Error while checking build/O2Physics/code-check for 3f44ff5 at 2026-09-30 03:12:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 2 errors and 3 warnings.'
Found 2 errors and 3 warnings.
++ [[ 2 -gt 0 ]]
++ cat /sw/BUILD/f553c9fc26e6f772ee711f532ca7a66b7c1140e1/O2Physics-code-check/errors.txt
PWGLF/TableProducer/GlobalEventProperties/eventShapeCoex.cxx:72:3: error: no header providing "o2::framework::Produces" is directly included [misc-include-cleaner,-warnings-as-errors]
PWGLF/TableProducer/GlobalEventProperties/eventShapeCoex.cxx:76:3: error: no header providing "o2::framework::Service" is directly included [misc-include-cleaner,-warnings-as-errors]
++ [[ 3 -gt 0 ]]
++ cat /sw/BUILD/f553c9fc26e6f772ee711f532ca7a66b7c1140e1/O2Physics-code-check/warnings.txt
PWGLF/TableProducer/GlobalEventProperties/eventShapeCoex.cxx:67:3: warning: function 'meanPt' should be marked [[nodiscard]] [modernize-use-nodiscard]
PWGLF/TableProducer/GlobalEventProperties/eventShapeCoex.cxx:71:8: warning: constructor does not initialize these fields: pdg [cppcoreguidelines-pro-type-member-init]
PWGLF/TableProducer/GlobalEventProperties/eventShapeCoex.cxx:137:22: warning: do not declare C-style arrays, use 'std::array' instead [modernize-avoid-c-arrays]
++ [[ 2 -gt 0 ]]
++ exit 1
--

Full log here.

[c-andrei]

Thanks for the review. I pushed a follow-up commit fixing the code-check findings (direct include of Framework/AnalysisHelpers.h plus three warnings); the output is unchanged. Could you please take another look and re-trigger the tests if needed?