feat(macos): add Canvas A2UI renderer

vendor/a2ui/CONTRIBUTING.md

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+# How to contribute to A2UI
+
+We'd love to accept your patches and contributions to this project.
+
+## Before you begin
+
+### Sign our Contributor License Agreement
+
+Contributions to this project must be accompanied by a
+[Contributor License Agreement](https://cla.developers.google.com/about) (CLA).
+You (or your employer) retain the copyright to your contribution; this simply
+gives us permission to use and redistribute your contributions as part of the
+project.
+
+If you or your current employer have already signed the Google CLA (even if it
+was for a different project), you probably don't need to do it again.
+
+Visit <https://cla.developers.google.com/> to see your current agreements or to
+sign a new one.
+
+### Review our community guidelines
+
+This project follows
+[Google's Open Source Community Guidelines](https://opensource.google/conduct/).
+
+## Contribution process
+
+### Code reviews
+
+All submissions, including submissions by project members, require review. We
+use GitHub pull requests for this purpose. Consult
+[GitHub Help](https://help.github.com/articles/about-pull-requests/) for more
+information on using pull requests.
+
+### Contributor Guide
+
+You may follow these steps to contribute:
+
+1. **Fork the official repository.** This will create a copy of the official repository in your own account.
+2. **Sync the branches.** This will ensure that your copy of the repository is up-to-date with the latest changes from the official repository.
+3. **Work on your forked repository's feature branch.** This is where you will make your changes to the code.
+4. **Commit your updates on your forked repository's feature branch.** This will save your changes to your copy of the repository.
+5. **Submit a pull request to the official repository's main branch.** This will request that your changes be merged into the official repository.
+6. **Resolve any linting errors.** This will ensure that your changes are formatted correctly.
+
+Here are some additional things to keep in mind during the process:
+
+- **Test your changes.** Before you submit a pull request, make sure that your changes work as expected.
+- **Be patient.** It may take some time for your pull request to be reviewed and merged.