feat: type the Pythia8 boundary with canonical quantities

[olantwin]

Stacked on #71. Beam energies, lifetime thresholds and production times become typed quantities; the hardcoded 299.792458 is replaced by the exact c-derived conversion (differences ≤1 ulp; 200-event validation run bitwise identical).

Summary by CodeRabbit

• Enhancements
  • Added unit-aware handling for beam energy, target geometry, interaction lengths, particle lifetimes, and position offsets.
  • Configuration values now support explicit units with sensible defaults.
  • Improved consistency of particle timing and position conversions across Pythia8 sources and benchmarks.
• Bug Fixes
  • Reduced the risk of incorrect simulations caused by ambiguous or mismatched measurement units.
  • Updated Pythia8 plugins and benchmarks to include the required unit support.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: a836134d-1468-4977-be0f-b9307ba07a39

📥 Commits

Reviewing files that changed from the base of the PR and between d98bf5c and cba7262.

📒 Files selected for processing (5)

• CMakeLists.txt
• benchmarks/pythia8_benchmark.cpp
• src/fixed_target_source.cpp
• src/pythia8_source.cpp
• src/pythia_common.hpp

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📝 Walkthrough

Walkthrough

Changes

Unit-aware Pythia integration

Layer / File(s)	Summary
Shared unit-aware Pythia helpers src/pythia_common.hpp	Pythia helpers now accept SHiP energy, length, and time quantities, converting values at Pythia and MCParticle boundaries.
Unit-aware source configuration src/pythia8_source.cpp, src/fixed_target_source.cpp	Source constructors and configuration parsing use typed beam, geometry, interaction-length, and lifetime parameters; interaction-point sampling is unit-aware.
Benchmark unit integration and linkage benchmarks/pythia8_benchmark.cpp, CMakeLists.txt	Benchmark calls use explicit energy and mm/c units, and Pythia plugins plus the benchmark link against SHiP::SHiPUnits.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• ShipSoft/aegir#34: Updates the related Phlex source integration and registration areas.
• ShipSoft/aegir#38: Modifies the shared Pythia helpers and their source and benchmark callers.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description states the goal but omits the required checklist items from the template.	Add the three checklist items for commit message, pre-commit checks, and CI checks, and mark each one.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: making the Pythia8 boundary use canonical quantities.
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.

✨ 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 feat/units-pythia8

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[github-actions]

Physics Metrics Comparison

1 configuration(s) have differences

• Configurations compared: 1
• With differences: 1

Configurations with Differences

comparison_physics-smoke

• Files compared: 1
• Histograms compared: 8
• Fits compared: 0
• Total differences: 4

Show differences

gun_st_nofield_smoke_validation.root:
  h_hit_edep.integral: 896 → 927 (Δ=3.46%)
  h_hit_z.integral: 896 → 927 (Δ=3.46%)
  h_hit_detector.integral: 896 → 927 (Δ=3.46%)
  h_hit_momentum.integral: 896 → 927 (Δ=3.46%)

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About this comparison

Physics metrics are extracted from ROOT files and compared.
Default comparison uses 5% tolerance for histogram statistics
and 3σ for fit parameters.

Metrics stored in git notes: refs/notes/ci/physics-metrics/<config>

[olantwin]

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[olantwin]

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[olantwin]

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