Refine benchmark tooling and add CI smoke run

[SaridakisStamatisChristos]

Summary

• ensure the benchmark CLI pins NumPy quantiles and gracefully resolves --class values
• add a "bench" extra with documentation updates and make the pytest accuracy guard capacity-aware
• run a lightweight benchmark smoke test in CI and publish its artifacts

Testing

• pytest -q

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https://chatgpt.com/codex/tasks/task_e_68e5159c6f408333889e74485d9179dc

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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