feat: Complete SwiftUI Implementation of v0.9 Basic Catalog

[sunnypurewal]

The breadth of the v0.9 spec requires this PR to be massive as it is meant to be a complete implementation. If there are best practices that you'd like me to follow to make the review of this PR manageable please let me know.

Description

This PR implements a SwiftUI renderer of the basic catalog components and functions defined in v0.9

The sample app demonstrates the union of the A2UI elements, the data model, and the rendered SwiftUI for each component. The basic catalog functions are also demonstrated on user input.

All Files Changed are new files. No files in the google:main branch have been edited.
I added a CHANGELOG as per the checklist.

Pre-launch Checklist

• I signed the CLA.
• I read the Contributors Guide.
• I read the Style Guide.
• I have added updates to the CHANGELOG.
• I updated/added relevant documentation.
• My code changes (if any) have tests.

If you need help, consider asking for advice on the discussion board.

[sunnypurewal]

Hi @gspencergoog @jacobsimionato @nan-yu and team. I'm reaching out to ask whether you believe this SwiftUI implementation of the v0.9 spec has value or what I can do to bring it to your attention. Perhaps I can collaborate with PR #706 to put together a comprehensive v0.8+v0.9 implementation? Please let me know if I should leave this PR open.

Thanks!

[zeroasterisk]

Hi @sunnypurewal, thanks for the v0.9 SwiftUI implementation — targeting the latest spec is great! 🙏

Same as the note on #706: we're moving community renderers to an ecosystem model where they live in their own repos. This gives you full ownership, faster iteration, and no bottleneck on our review queue.

Recommended path:

1. Publish as your own repo (e.g. sunnypurewal/a2ui-swiftui)
2. Open a PR to add it to our Ecosystem Renderers page
3. We're building conformance tests (Program to accept and bless community-maintained renderers #681) to help verify spec compatibility across all renderers

Note: @BBC6BAE9 also built a SwiftUI renderer (PR #706) — collaboration could be valuable since you're both targeting the same platform.

The ecosystem page is at a2ui.org/ecosystem/renderers — we'd love to list a SwiftUI renderer there!

[zeroasterisk]

NOTE: it is possible we could "graduate" from a community renderer to the core A2UI repo, with maintenance guarantees and conformance testing. We are currently not staffed to be able to make headway on this, right now and I want to unblock you. Please self host and link in docs.