[Bug] LayoutFooter Properties preview renders no working component

## Affected page

https://astryx.atmeta.com/components/LayoutFooter?tab=properties

## Steps to reproduce

1. Open the affected component page in Chromium 149.0.7827.55 on macOS.
2. Open the **Properties** tab.
3. Observe the preview before changing any props.
4. Exercise the available controls where applicable.

## Expected behavior

A visible footer region renders in a minimal Layout scaffold.

## Actual behavior

The preview stage is completely empty except for the Show code control.

No unrelated console or page errors were observed during the fresh-context reproduction.

## Diagnosis

The subcomponent has no child content or parent-layout playground wrapper, so its standalone region is empty.

The likely fix is local to `packages/core/src/Layout/LayoutFooter.doc.mjs` and/or its focused docsite preview coverage; it does not require a new public API or design direction. This is an atomic, independently closable slice of #2008.

## Acceptance criteria

- [ ] Add representative footer content and a minimal Layout playground wrapper.
- [ ] The preview renders a visible footer on first load.
- [ ] A relevant footer control such as divider or height produces an observable update.
- [ ] Add focused preview/data-extraction coverage.

Start with packages/core/src/Layout/LayoutFooter.doc.mjs and reproduce the empty Properties preview at the affected LayoutFooter page. Inspect the focused docsite preview/data-extraction coverage, then verify that representative footer content, a minimal Layout wrapper, and a footer control update produce the expected visible result on first load.

Written by the indexing model from the issue text.