Prototype for discussion, not intended to merge as-is. The package is private and the API is a sketch.

Changes

• Adds @astrojs/ui, an integration that lets components use plain TypeScript with signals bound to the DOM. It's progressive enhancement: the server renders HTML, and <script scoped> attaches to it. The aim is a small, framework-free model that's easy to write correctly, including for agents.
• <script scoped> runs once per component instance. Its scope(selector) sees only that instance's markup: not sibling instances, not child component internals, but it does see content passed into a child's <slot>. It throws when nothing matches. The integration parses .astro source with @astrojs/compiler-rs before compilation, wraps each instance and slot in comment markers, and turns the scoped script into a normal bundled script that registers with a small client runtime (~3.6 KB gzipped, including alien-signals).
• signal() in frontmatter renders its value with a marker in {count} and value={count}. The client revives each signal once by id, so one signal passed to several components stays shared in the browser, with no script needed. value and checked bindings are two-way.
• scope.list(container, template, row => item) adopts server-rendered rows and clones a <template> for new ones. Reordering and filtering use keyed reconciliation ported from zebra's List.

---
import { signal } from '@astrojs/ui';
const count = signal(Astro.props.start ?? 0);
---
<output>{count}</output>
<button class="inc">+</button>

<script scoped>
  const count = scope('output').text();
  scope('.inc').on('click', () => count(count() + 1));
</script>

• Adds a 7GUIs Cells example (test/fixtures/cells): a 26 × 100 spreadsheet where each cell is a formula signal plus a computed value, with a formula engine shared by server and client so the first render has real values. About 6 KB gzipped of client JS.

Open questions (instances added after load, row disposal, build-time selector checks) are listed in the package README.

Testing

• Unit tests for the source transform, the runtime's instance/slot ownership, signal revival and list adoption (using linkedom), and list reconciliation.
• Playwright tests for Cells: server-rendered values, edits propagating through dependent cells, cycle detection and recovery, error values, and formula bar and keyboard editing.
• Playwright tests against a demo fixture: per-instance counters, a card that must not see slotted todo checkboxes, a signal shared between two script-less components, and adding/filtering/sorting/removing todos.

Docs

• No docs update, because this is an unpublished prototype.