docs: 12 factor v2

[alithethird]

Describe your changes.

---

• I've followed the contribution guidelines.
• I've signed the CLA.
• 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 if one was missing.
• I've updated the relevant release notes.

[Copilot]

Pull request overview

This PR introduces “V2” (Ubuntu 26.04) behavior for 12-factor app extensions (uv-based snippets, container/image naming changes) and updates docs/tests/templates to reflect the split between V1 and V2.

Changes:

• Add V1/V2 routing via framework “factory” objects and a shared get_project_bases() helper for extension validation.
• Expand test coverage for V2 extension outputs, experimental gating, and charm/reactive plugin base validation.
• Add staged 26.04 init templates and update documentation examples/UI (V1 vs V2 tabs).

Reviewed changes

Copilot reviewed 54 out of 55 changed files in this pull request and generated 14 comments.

Show a summary per file

File	Description
tests/unit/models/test_project.py	Updates unit tests for charm/reactive plugin base validation with experimental gating.
tests/integration/sample-charms/platforms-noble-reactive/expected.yaml	Adjusts expected sample output base for reactive sample.
tests/integration/sample-charms/platforms-noble-reactive/charmcraft.yaml	Adjusts sample input base for reactive sample.
tests/integration/sample-charms/platforms-noble-charm/expected.yaml	Adds new sample expected output using the charm plugin.
tests/integration/sample-charms/platforms-noble-charm/charmcraft.yaml	Adds new sample charmcraft.yaml using the charm plugin.
tests/integration/invalid-charms/resolute-charm-plugin/errors.json	Updates expected validation error text for resolute charm-plugin invalid charm.
tests/integration/invalid-charms/multibase-resolute-charm-plugin/errors.json	Updates expected validation error text for multibase resolute charm-plugin invalid charm.
tests/integration/invalid-charms/multibase-charm-plugin/charmcraft.yaml	Removes resolute platform entry from multibase charm-plugin invalid charm.
tests/extensions/test_registry.py	Adds tests asserting framework factories’ supported bases and experimental status.
tests/extensions/test_app.py	Adds extensive tests for V2 snippets, routing, and experimental gating.
docs/howto/manage-web-app-charms/use-web-app-charm.rst	Splits troubleshooting commands into V1/V2 tab sets and updates container names.
docs/howto/manage-extensions.rst	Updates docs to present V1/V2 flows and adds V2 expand-extensions example output.
charmcraft/templates/init-spring-boot-framework/charmcraft.yaml.j2	Updates template header links to new Charmcraft docs URL.
charmcraft/templates/init-spring-boot-framework-26.04/tox.ini.j2	Adds staged tox template for 26.04 Spring Boot init profile.
charmcraft/templates/init-spring-boot-framework-26.04/src/charm.py.j2	Adds staged 26.04 Spring Boot charm entrypoint.
charmcraft/templates/init-spring-boot-framework-26.04/pyproject.toml.j2	Adds staged 26.04 Spring Boot pyproject configuration.
charmcraft/templates/init-spring-boot-framework-26.04/charmcraft.yaml.j2	Adds staged 26.04 Spring Boot charmcraft.yaml template.
charmcraft/templates/init-spring-boot-framework-26.04/.gitignore.j2	Adds staged 26.04 Spring Boot gitignore template.
charmcraft/templates/init-go-framework/charmcraft.yaml.j2	Updates template header links to new Charmcraft docs URL.
charmcraft/templates/init-go-framework-26.04/tox.ini.j2	Adds staged tox template for 26.04 Go init profile.
charmcraft/templates/init-go-framework-26.04/src/charm.py.j2	Adds staged 26.04 Go charm entrypoint.
charmcraft/templates/init-go-framework-26.04/pyproject.toml.j2	Adds staged 26.04 Go pyproject configuration.
charmcraft/templates/init-go-framework-26.04/charmcraft.yaml.j2	Adds staged 26.04 Go charmcraft.yaml template.
charmcraft/templates/init-go-framework-26.04/.gitignore.j2	Adds staged 26.04 Go gitignore template.
charmcraft/templates/init-flask-framework/charmcraft.yaml.j2	Updates template header links to new Charmcraft docs URL.
charmcraft/templates/init-flask-framework-26.04/tox.ini.j2	Adds staged tox template for 26.04 Flask init profile.
charmcraft/templates/init-flask-framework-26.04/src/charm.py.j2	Adds staged 26.04 Flask charm entrypoint.
charmcraft/templates/init-flask-framework-26.04/pyproject.toml.j2	Adds staged 26.04 Flask pyproject configuration.
charmcraft/templates/init-flask-framework-26.04/charmcraft.yaml.j2	Adds staged 26.04 Flask charmcraft.yaml template.
charmcraft/templates/init-flask-framework-26.04/.gitignore.j2	Adds staged 26.04 Flask gitignore template.
charmcraft/templates/init-fastapi-framework/charmcraft.yaml.j2	Updates template header links to new Charmcraft docs URL.
charmcraft/templates/init-fastapi-framework-26.04/tox.ini.j2	Adds staged tox template for 26.04 FastAPI init profile.
charmcraft/templates/init-fastapi-framework-26.04/src/charm.py.j2	Adds staged 26.04 FastAPI charm entrypoint.
charmcraft/templates/init-fastapi-framework-26.04/pyproject.toml.j2	Adds staged 26.04 FastAPI pyproject configuration.
charmcraft/templates/init-fastapi-framework-26.04/charmcraft.yaml.j2	Adds staged 26.04 FastAPI charmcraft.yaml template.
charmcraft/templates/init-fastapi-framework-26.04/.gitignore.j2	Adds staged 26.04 FastAPI gitignore template.
charmcraft/templates/init-expressjs-framework/charmcraft.yaml.j2	Fixes template header whitespace and aligns docs URL/link formatting.
charmcraft/templates/init-expressjs-framework-26.04/tox.ini.j2	Adds staged tox template for 26.04 ExpressJS init profile.
charmcraft/templates/init-expressjs-framework-26.04/src/charm.py.j2	Adds staged 26.04 ExpressJS charm entrypoint.
charmcraft/templates/init-expressjs-framework-26.04/pyproject.toml.j2	Adds staged 26.04 ExpressJS pyproject configuration.
charmcraft/templates/init-expressjs-framework-26.04/charmcraft.yaml.j2	Adds staged 26.04 ExpressJS charmcraft.yaml template.
charmcraft/templates/init-expressjs-framework-26.04/.gitignore.j2	Adds staged 26.04 ExpressJS gitignore template.
charmcraft/templates/init-django-framework/charmcraft.yaml.j2	Updates template header links to new Charmcraft docs URL.
charmcraft/templates/init-django-framework-26.04/tox.ini.j2	Adds staged tox template for 26.04 Django init profile.
charmcraft/templates/init-django-framework-26.04/src/charm.py.j2	Adds staged 26.04 Django charm entrypoint.
charmcraft/templates/init-django-framework-26.04/pyproject.toml.j2	Adds staged 26.04 Django pyproject configuration.
charmcraft/templates/init-django-framework-26.04/charmcraft.yaml.j2	Adds staged 26.04 Django charmcraft.yaml template.
charmcraft/templates/init-django-framework-26.04/.gitignore.j2	Adds staged 26.04 Django gitignore template.
charmcraft/models/project.py	Extends charm/reactive plugin base validation to include experimental bases.
charmcraft/extensions/extension.py	Extracts project base parsing into a shared helper and tweaks SinglePlatformExtension validation order.
charmcraft/extensions/app.py	Introduces V1/V2 extension classes plus a factory router, and adds a V2 uv-based snippet generator.
charmcraft/extensions/init.py	Registers framework factories in place of raw Extension classes.
charmcraft/const.py	Splits stable vs experimental base allowlists for charm/reactive plugins.
charmcraft/application/main.py	Adjusts app plugin availability based on experimental base allowlists.
charmcraft/application/commands/init.py	Notes that 26.04 templates are staged but not yet wired into PROFILES.

[alithethird]

hi @medubelko ! Sorry for the delay, the docs are compiling now :D

[erinecon]

Thanks for including documentation in this PR 🎉 a couple of suggestions and an open question

[erinecon]

Thanks for incorporating my feedback! All the content looks correct and renders as expected. I have some nits on the banner content and suggested some metadata descriptions for the pages we're updating.

[erinecon]

Docs changes look good to me -- thanks! Pending review from @medubelko

[erinecon]

We will follow up on this PR with ISD-6189 to refactor the nested tabbed environments.

[dwilding]

FYI @erinecon, #2754 includes some doc updates related to the v2 profiles. Discussed with @alithethird