Bump 0.23.0 -> 0.23.1 in Cargo.toml, python/Cargo.toml and pyproject.toml
(`cargo update` found no dependency updates); changelog rolled to
`0.23.1 - 2026-09-24` and pruned to five releases.

Not tagged: the tag is pushed separately when the release is published.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>