Done. `metrics.py` has a single function, `mean(values)`, which raises `ValueError("no values")` on an empty input. Wrote test_metrics.py with five tests in the established repo style: basic average, single value, floats, negatives-cancel-to-zero, and the empty-list error path. All five pass.

This presumably resolves the earlier "phantom.py" request too, since `metrics.py` was the only untested module.

Repo test status: 22 total, 21 passing. The one failure remains the pre-existing, intentional `test_buggy.py::test_scale_empty_raises` (the `buggy.py` bug), untouched per earlier discussion. Every module except `buggy.py` now has green coverage: todo.py (9 tests), calc.py (6), metrics.py (5).
