Fix nonpositive benchmark iteration counts

[KingEmma7]

Summary

Reject zero and negative --n-permutations and --n-resamples values before the benchmark starts, with argparse errors and tests covering invalid input, valid counts and unchanged defaults.

Type

• Bug fix

Validation

make check passed: 490 tests, lint, all 14 audit scripts and the CLI smoke tests. The invalid-count regressions fail against the original CLI.

Linked issue

Closes #618
Closes #617

[vercel]

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[github-actions]

Hey @KingEmma7, your first PR on Fair Code, that's awesome, thank you.

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[github-actions]

@yakew7 @ahmdkaml - new PR touching a path you own, please review.