Add a dedicated writable `/app-data` directory to the v2 12-factor framework extensions for Django, ExpressJS, FastAPI, Flask, Go, and Spring Boot on Ubuntu 26.04 and later. The directory is created by a shared v2 mixin and owned by the `_daemon_` UID/GID sourced from `SUPPORTED_GLOBAL_USERNAMES`. The v1 extension classes are unchanged, with explicit unit-test regression coverage confirming that v1 expansion does not include the new part. Coverage includes all six v2 extension expansions and Ubuntu 26.04 spread ownership/writability checks. The FastAPI spread fixture additionally performs an application-level `POST` that writes deterministic content to `/app-data`, then verifies the file content and `_daemon_` ownership. Validation performed: - 205 targeted extension unit tests passed - Targeted Ruff lint and formatting checks passed - Targeted `ty` checks passed - `make lint-shellcheck` passed - `git diff --check` passed Full spread builds were not run locally. --- - [x] I've followed the [contribution guidelines](https://github.com/canonical/rockcraft/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. - [ ] I've updated the relevant release notes.