chore(shipgen): remove unused Pythia6Generator

[olantwin]

Summary

• Audit (follow-up to tpythia6Generator error in makeCascade.py #1272 / fix(shipgen): make tPythia6Generator concrete again (#1272) #1274) found Pythia6Generator (no t) has zero callsites: no Python script constructs it, no C++ caller exists, no docs / README / CHANGELOG reference it, no CI workflow exercises it. Its .cxx and .h have only seen cosmetic refactors since the 2008 import.
• It also has two latent bugs that fix(shipgen): make tPythia6Generator concrete again (#1272) #1274 was about to paper over: it inherits abstract Init overloads from SHiP::Generator without overriding them, and its default constructor leaves raw FILE* / Char_t* pointers indeterminate, crashing the destructor on default-construction.
• Delete the class rather than fix it. tPythia6Generator is unaffected (it is actively used by macro/makeCascade.py and run_simScript.py --Pythia6). The ROOTEGPythia6 dependency stays for tPythia6Generator.

Test plan

• pixi run build succeeds with no unresolved references
• hasattr(ROOT, 'Pythia6Generator') returns False after loading libShipGen
• tPythia6Generator source file is untouched (git diff master -- shipgen/tPythia6Generator.* is empty); independent of this PR, it still has the tpythia6Generator error in makeCascade.py #1272 abstract-class bug that fix(shipgen): make tPythia6Generator concrete again (#1272) #1274 fixes
• No straggler reference found by full-tree grep
• CI passes

Independent of #1274 — neither PR blocks the other.

Summary by CodeRabbit

• Removed
  • Removed the legacy text-format event reader. An alternative implementation remains available for users who require this functionality.

[coderabbitai]

📝 Walkthrough

Walkthrough

The PR removes the unused legacy Pythia6Generator class by deleting its header (Pythia6Generator.h, 131 lines) and implementation (Pythia6Generator.cxx, 117 lines), dropping Pythia6Generator.cxx from the CMake ShipGen library sources, updating the ROOT LinkDef.h dictionary pragmas to replace Pythia6Generator with tPythia6Generator and add SHiP namespace directives, and recording the removal in CHANGELOG.md.

Changes

Remove legacy Pythia6Generator

Layer / File(s)	Summary
Build system and ROOT dictionary cleanup shipgen/CMakeLists.txt, shipgen/LinkDef.h, CHANGELOG.md	Pythia6Generator.cxx is dropped from the ShipGen CMake SOURCES list. In LinkDef.h, the #pragma link C++ class Pythia6Generator is replaced with #pragma link C++ class tPythia6Generator and new directives for the SHiP namespace and SHiP::Generator. The changelog records the removal under ### Removed.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Possibly related PRs

• ShipSoft/FairShip#1225: Modifies Pythia6Generator::ReadEvent in shipgen/Pythia6Generator.cxx (the same implementation file entirely deleted by this PR).

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and concisely summarizes the main change: removing the unused Pythia6Generator class from the shipgen module.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	Pull request description provides comprehensive context including detailed summary of rationale, technical justification, test plan verification, and impact clarification.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/remove-unused-pythia6generator

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[olantwin]

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[coderabbitai]

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