[andrewkolos]

Adds dart/a2ui_flutter/a2ui_explorer, the renderer's gallery app, as section 9 of the framework adapter blueprint describes.

Contents

• A three column layout, as specified by section 9 of the blueprint. The left lists the v0.9 basic catalog examples. The center shows the selected example's surfaces above its messages, with Advance, Run all and Reset. The right shows each surface's live data model above a log of actions and errors.
• Selecting an example renders all of its messages. Reset starts it over for stepping through.
• A demo catalog: the basic catalog plus the custom components the examples name, CustomGrid and CustomSlider.
• The app bundles the specification's examples through a link, assets/examples.
• macOS and web, like samples/client/flutter/restaurant_finder.

gallery.mp4

Testing

A test runs and renders every example and fails on any error, and another checks that an error reported during a surface's first build reaches the log. Integration tests drive the examples through what section 9 lists: static rendering, layout, two-way binding, reactive updates, and action context inside a template, plus Advance, Run all and Reset. No example has an action inside a List template, so the template test uses a Column's.

Pre-launch Checklist

One time:

• I signed the CLA.
• I read the Contributors Guide.
• I read the Style Guide.
• I have built and run at least one sample.

For this PR:

• I have updated the relevant CHANGELOG.md file.
• I updated/added relevant documentation.
• My code changes (if any) have tests.
• If my branch is on a fork, I have verified that scripts/e2e_test.sh passes.

_{ℹ️ Initial text drafted by agent}