Enable code coverage with Codecov

[namurphy]

Summary

• Add a coverage nox session (e.g., nox -s coverage-3.12) that runs pytest with --cov and writes coverage.xml. It uses the editable install from uv sync so that file paths in the report point to src/.
• Add pytest-cov to the test dependency group and update uv.lock.
• Configure coverage.py in [tool.coverage] in pyproject.toml: branch coverage, source_pkgs = ["pyfaradaycup"], path mapping, and exclusions for lines like if TYPE_CHECKING:.
• In CI, switch the Python 3.12 Ubuntu job from tests-3.12 to coverage-3.12. This triggers the existing Codecov upload step, which runs for any session whose name contains cov.
• Add .github/codecov.yml so that Codecov status checks are informational and never fail a PR.

Running nox -s coverage-3.12 locally reports 55% coverage.

Manual steps after merging

• Enable PlasmaPy/pyfaradaycup on https://app.codecov.io. This requires the Codecov GitHub App to be installed for the PlasmaPy organization.
• Add the Codecov upload token as the repository secret CODECOV_TOKEN.
• Optional: add a Codecov badge to README.md.

🤖 Generated with Claude Code

[namurphy]

We'll probably want to adjust these later so that checks fail if either the coverage goes down by a non-negligible amount, or if coverage falls below a floor value. That can lead to some erroneous failures, but is generally helpful.