chore: ignore .ruff_cache

The lint tooling added alongside the test suite writes a cache
directory that .gitignore did not cover, unlike .pytest_cache beside
it. Harmless in practice -- ruff drops a self-ignoring .gitignore into
its own cache -- but the repository should account for its own tooling.

Author: bgriffen

.gitignore

@@ -14,8 +14,9 @@ dist/
 venv/
 env/
 
-# Test / coverage artefacts
+# Test / lint / coverage artefacts
 .pytest_cache/
+.ruff_cache/
 .coverage
 coverage.xml
 htmlcov/