**Docs** Updates the 12-factor extension docs for Ubuntu 26.04 and fixes errors found by checking each claim against the extension code and real packs. * How-to guides: - set-up-web-app-rock now covers --base ubuntu@26.04, the Spring Boot init, extra files, extra debs, and the experimental extensions. - use-web-app-rock  now has Express and Spring Boot tabs. - The Express build-base guide quotes npm-node-version  values ( "20.12" ). - The migration guide fixes  rockcraft list-extensions and the uv/Poetry paragraph. * Reference pages (Flask, Django, FastAPI, Go, Express, Spring Boot): - Flask and FastAPI only include their default files, so other modules must be listed in  prime. - The rock name must equal the last element of the Go module path. - On  bare  rocks, extra packages must be slices, because mixing packages and slices fails. - The Express docs now say that  files  and  .npmignore  apply, and that a  build  script's  dist  is always included. - The <name> directory has - replaced by  _ . **Small Code fix** On Ubuntu 26.04, the Flask and Django extensions started gunicorn with  -k sync . The V1 extensions (22.04 and 24.04) use  -k [ sync ] , and paas-charm rewrites that value itself. This PR removes the  _worker_class_template = "{}"  override in  gunicorn.py , so V2 uses the same form as V1. I updated the two unit tests that expected the old form. --- - [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/). - [x] I've successfully run `make lint && make test`. - [x] 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.