feat(eval): add parallel task execution support via max_workers in eval_set

[ujjwalredd]

Summary

Adds optional parallel task execution to eval_set() via a max_workers: int = 1 parameter and an --max-workers flag in inspect-robots eval-set.

Changes

• Updated eval_set() in src/inspect_robots/eval.py to validate max_workers >= 1 and dispatch tasks concurrently via ThreadPoolExecutor when max_workers > 1.
• Added --max-workers CLI option to inspect-robots eval-set in src/inspect_robots/cli.py.
• Added unit tests in tests/test_eval_orchestration.py verifying validation and parallel task collection while maintaining 100% test coverage.
• Updated CHANGELOG.md.

Checklist

• pre-commit install done; hooks pass
• Tests added/updated
• Coverage stays at 100% (pytest --cov)
• ruff check . and ruff format --check . pass
• mypy passes (strict)
• CHANGELOG.md updated under "Unreleased"
• Public API changes are reflected in inspect_robots.__all__ and the API-snapshot test
• Core stays NumPy-only (new deps are optional extras, lazily imported)

Related

Closes #361