feat(g4): add merge_across_tracks, one hit per placement

[matclim]

Checklist

• Commit message follows conventional commits
• Pre-commit checks pass (pixi run lint)
• CI checks pass

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[github-actions]

Physics Metrics Comparison

All configurations match reference (no significant differences)

• Configurations compared: 1
• Matching reference: 1

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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>

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