- Create `pub.p5i` files during publisher addition and transaction commits to maintain compatibility with older IPS versions.
- Introduce `create_pub_p5i_file` method in `FileBackend` for generating structured `pub.p5i` JSON files.
- Update `add_publisher` and `Transaction::commit` methods to handle `pub.p5i` creation logic.
- Add tests to verify `pub.p5i` file generation in various scenarios.
