fix: resolve formatting, regression logic, and pytest defaults

- Run ruff format on all benchmark files (fixes CI lint job)
- Fix check_regression() substring ambiguity: ordered keyword matching
  so "latency_improvement_pct" is correctly classified as higher-is-better
- Update stale comments in conftest.py referencing wrong fixture
- Add pytest addopts to skip benchmark/slow/stress markers by default

pyproject.toml

@@ -65,6 +65,7 @@ quote-style = "double"
 [tool.pytest.ini_options]
 testpaths = ["tests"]
 pythonpath = ["."]
+addopts = "-m 'not benchmark and not slow and not stress'"
 markers = [
     "benchmark: scale/performance benchmark tests",
     "slow: tests that take more than 30 seconds",