Pandora bugfix - Shower merging algorithm - Production

[aliciavr]

Restore the shower merging algorithm in another place to avoid inconsistent state of PFParticles (-
1 track scores).

Description

Please provide a detailed description of the changes this pull request introduces.

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$${\color{blue}\bf{\textrm{IMPORTANT UPDATE Feb 2nd 2026:}}}$$ If you are making a PR which is intended as a patch for the CURRENT production for gen 2 SBND analyses, you must make two PRs: one for develop and one for the production/sbnd-gen2 branch.

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Checklist

• Added at least 1 label from available labels.
• Assigned at least 1 reviewer under Reviewers,
• Assigned all contributers including yourself under Assignees
• Linked any relevant issues under Developement
• Does this PR affect CAF data format? If so, please assign a CAF maintainer (PetrilloAtWork or JosiePaton) as additional reviewer.
• [] Does this affect the standard workflow?
• Is this PR a patch for the ongoing production? If so, separate PR must also be made for production/v10_06_00 branch!
  Done in production/sbnd-gen2

Relevant PR links (optional)

Does this PR require merging another PR in a different repository (such as sbnanobj/sbnobj etc.)?

Link(s) to docdb describing changes (optional)

Is there a docdb describing the issue this solves or the feature added?

[AndyChappell]

Presumably this one should be targeting a merge into the production branch, not develop?

[aliciavr]

@AndyChappell Yes, they seem to be the same branch