StateTrace 0.3.1

pip install statetrace-sc==0.3.1 (import name statetrace).

0.3.0 reformatted; no code change. The CI lint job had failed ruff format --check on two
files since the v0.2.1 tag, and the release workflow's publish job depends on it, so neither
0.2.1 nor 0.3.0 reached PyPI. 0.3.1 is the first published 0.3 release; its behaviour, API and
record columns are those of 0.3.0 (31e1856, the commit installed from the git tag on
Bridges-2 for the benchmark's application stages).

Fixed

• Formatting of robustness.py and tests/test_robustness_aggregation_features.py to the
  pinned ruff (0.6.9); CI and the release workflow pass again.