Enforce the existing 100% coverage of the Python layer in CI

[tschm]

Closes #238

scs/py/__init__.py is at 100% statement coverage today, but nothing measures it, so a regression would land silently.

The reason it was never wired up is a real trap: --cov=scs collects nothing when pytest runs from the repo root. scs/ has no __init__.py, so coverage resolves the name to that namespace-package directory rather than to the installed package, and reports 0% with No data was collected. source_pkgs says scs is an importable package name, so coverage follows the import to site-packages instead.

• [tool.coverage.run] source_pkgs = ["scs"] in pyproject.toml
• [tool.coverage.report] fail_under = 100, so the CI invocation is a bare pytest --cov with no inline flags
• pytest-cov added to build_openmp's pixi deps

Gate added to build_openmp only; the other test jobs run plain pytest, where the coverage config is inert.

Also gitignores the coverage data files — the existing entry was python/.coverage, from the pre-meson layout.

Verified locally from the repo root:

site-packages/scs/__init__.py   114   0   100%
Required test coverage of 100.0% reached. Total coverage: 100.00%
396 passed, 67 skipped

The threshold is safe cross-platform: test_scs_coverage.py mocks sys.platform for darwin, linux and win32, so _resolve_auto's branches are covered regardless of the runner.

🤖 Generated with Claude Code

[bodono]

Thanks for the work, but as noted on #238 we're not adding a coverage gate on the Python layer. Closing.