feat(ui): add bindable component metadata contracts

[artiphishle]

Summary

Implements #49.

Adds bindable metadata contracts to the neutral UiComponentMeta model so UI packages can declare which component props and events are data-bindable.

Adds:

• UiBindableValueType
• UiBindableValueFieldMeta
• UiBindableValueMeta
• UiBindablePropMeta
• UiBindableEventPayloadMeta
• UiBindableEventMeta
• UiComponentBindingMeta
• optional bindings field on UiComponentMeta

Also adds serialization tests for:

• bindable Button props/events
• bindable List collection item fields/events
• metadata registries with bindable props
• extension package manifests with bindable props/events

Scope intentionally excluded

• no actual component instance binding model from 02 — Replace current bindings.ts with shared component data-binding contracts #48
• no replacement of src/bindings.ts
• no ZORA metadata migration
• no Studio UI changes
• no Runtime changes

Verification

I could not run local validation from this session because the environment cannot reliably clone/resolve GitHub repositories.

Please run before merge:

bun install
bun run build
bun run lint:fix
bun run test

Notes

This PR extends the generic component metadata model from #46. It does not define concrete app/component instance bindings; that remains the next step in #48.