Fixes #1390 Warns when a part's `source` is an absolute path or a relative path that goes above the project dir, since those usually don't exist in a managed build. The check only looks at the path itself. URLs and `git@host:repo` sources are skipped. It uses `warnings.warn`, like `PartitionUsageWarning`. Known limitations: - With craft-application's experimental monorepo mode (git build root), `../shared` is valid but still warns. The app could filter the warning in that mode, or craft-parts could take a wider root for the check. Let me know which you'd prefer and I'll add it. - A symlink inside the project that points outside isn't caught. - craft-application doesn't route Python warnings, so it shows up as a plain `SourceOutsideProjectWarning: ...` line. Tested with snapcraft 9.2 in destructive mode and in a simulated managed run, where the `../../../` case from the issue warns and then fails the same way. I ran ruff, ty, codespell and the unit tests in a container instead of `make lint && make test`. --- - [x] I've followed the [contribution guidelines](https://github.com/canonical/craft-parts/blob/main/CONTRIBUTING.md). - [x] I've signed the [CLA](http://www.ubuntu.com/legal/contributors/). - [ ] I've successfully run `make lint && make test`. - [ ] I've added or updated any relevant documentation. - [ ] In documents I changed, I [added a meta description](https://canonical-starflow.readthedocs-hosted.com/how-to/add-a-page-meta-description/) if one was missing. - [x] I've updated the relevant release notes.