fix: validate n >= 1 in the MMLU/GSM8K slice loaders (fixes #8)

[Nitjsefnie]

Fixes #8.

Both load_mmlu_slice and load_gsm8k_slice now raise ValueError("n must be >= 1, got {n}") at the very top — before _cache_path(...) (which mkdirs the cache root) and before the function-local from datasets import load_dataset — so the guard is reachable with no download, no network, and no side effects, per the issue's ordering requirement.

Tests: four parametrized cases in tests/test_metrics.py (n=0 and n=-3 for each loader), pytest.raises(ValueError), no existing test touched.

Verification

• ruff check ., ruff format --check ., mypy src, pytest -q all green (56 tests, 2 pre-existing unrelated skips — torch/matplotlib extras).
• Mutation-proven twice (implementer, then independently re-run before opening this PR): removing only the mmlu guard makes exactly the two mmlu tests fail — and the failure mode is itself informative: ModuleNotFoundError: No module named 'datasets' at metrics.py:584, i.e. without the guard the call sails past the cache check into the gated import, which is precisely the no-network property the guard exists to protect. gsm8k tests unaffected by the mmlu mutation; restore → all green.

One boundary note: the guards reject n < 1 but deliberately not non-int types (out of #8's stated scope), and score_mmlu([], []) still returns 0.0 — the issue fixes the misleading-0% at the loader boundary, which is what this does.

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🤖 This PR was written with AI assistance (implementation: Kimi K3; verification and review: Claude), directed by a human-supervised workflow.

[bamdadd]

Thanks Peter — verified locally: ruff, mypy, and the full metrics suite all green, and the guard fires before any cache/datasets access as your test asserts. The ValueError("n must be >= 1, got {n}") reads cleanly and doesn't touch the valid-n path, so existing behavior is unchanged. Merging.

One small follow-up if you're up for it: an accept-path test (n=1 against a pre-seeded tmp cache_dir) would pin the happy path without needing network — nice complement to the reject cases here. Not blocking. Appreciate the fix!