feat: type the GENIE and particle-gun boundaries

[olantwin]

Stacked on #72. GENIE SI→canonical conversions become derived quantities (exact powers of ten); gun config is typed via get_quantity with the Philox stream untouched. 100-event gun_only output bitwise identical to origin/main.

Summary by CodeRabbit

• Improvements
  • Added unit-aware handling for beam energies, distances, angles, momenta, lifetimes, and event coordinates.
  • Configuration values now accept explicit physical units, reducing ambiguity and preventing unit-conversion errors.
  • Updated particle generation, fixed-target interactions, Pythia8 simulations, and GENIE event conversion to use consistent units.
  • Improved benchmark configuration with correctly scaled energy and lifetime values.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 97a9f5c6-f6f5-4dd4-8985-dcf7d7960876

📥 Commits

Reviewing files that changed from the base of the PR and between c5d0220 and 9218e28.

📒 Files selected for processing (7)

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

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📝 Walkthrough

Walkthrough

The PR replaces raw generator configuration scalars with SHiP unit quantities across particle gun, fixed-target, Pythia8, GENIE, and benchmark paths. Shared Pythia helpers perform explicit unit conversions, and required SHiP unit libraries are added to plugin and benchmark linkage.

Changes

Unit-aware event generation

Layer / File(s)	Summary
Pythia unit contracts src/pythia_common.hpp	Beam energy, lifetime thresholds, extraction offsets, and time conversion use typed SHiP quantities.
Source quantity integration src/particle_gun_source.cpp, src/fixed_target_source.cpp, src/pythia8_source.cpp, src/genie_reader_source.cpp, CMakeLists.txt	Source configuration and generation paths use explicit momentum, angle, energy, length, and time units, with required unit-library linkage.
Benchmark unit wiring benchmarks/pythia8_benchmark.cpp, CMakeLists.txt	Benchmark beam and lifetime arguments use typed units, and the executable links against SHiP units.

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

Possibly related PRs

• ShipSoft/aegir#34: Earlier source and wiring changes overlap the updated event generator implementations.
• ShipSoft/aegir#38: Shared Pythia helpers and benchmark paths are modified in both PRs.
• ShipSoft/aegir#72: Directly overlaps the unit typing and Pythia/CMake linkage changes.

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description omits the required checklist sections and doesn't cover commit message, lint, or CI status.	Rewrite it using the repo template and add the checklist items, including commit message, pixi run lint, and CI status.
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise and matches the PR's main focus on typed GENIE and particle-gun boundaries.
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-readers

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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: 919 → 969 (Δ=5.441%)
  h_hit_z.integral: 919 → 969 (Δ=5.441%)
  h_hit_detector.integral: 919 → 969 (Δ=5.441%)
  h_hit_momentum.integral: 919 → 969 (Δ=5.441%)

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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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