Make GeneratorHybrid a singleton

[jackal1-66]

By making the generators list static we can provide info to the other generators in cocktails.
Currently each generator runs without knowing the information from the others in the hybrid configuration, but this will allow them to "communicate".
I tested it using an external generator which uses the GetParticles().size() as a baseline to change the number of generated particles (boxgen). It's only the first piece of the puzzle and more work is required to take care of different scenarios (like parallel simulations).

[github-actions]

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[alibuild]

Error while checking build/O2/fullCI_slc9 for 4e4c801 at 2025-10-17 07:18:

## sw/BUILD/O2Physics-latest/log
Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:
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Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:
Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:
Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:
Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:
Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:
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Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:
Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:
[0 more errors; see full log]

Full log here.

[sawenzel]

This effectively makes the HybridGenerator a singleton since you can't have multiple instances in isolation (the generator members are global). I think it would then be better to just explicitly make the class a singleton to avoid static function annotations (or a mix between static and non-static).

Is there a concrete motivation behind this dev?

[jackal1-66]

The idea is to make the generators declared in the hybrid configuration "environment"-aware. I thought of making it similar to the embedding mechanism we have right now (this would be the start of the development). An example of usage is shown in this external generator AliceO2Group/O2DPG#2159 which is the motivation behind this development.

[jackal1-66]

Would this work? I tried to keep it as simple as possible. I ran some tests with o2-sim standalone, o2-sim-dpl-eventgen and with a full O2DPG workflow and didn't encounter any issue.