feat: skeleton component (Web Awesome parity)

[eddyloewen]

Goal

Add skeleton to the library, following the Web Awesome anatomy of wa-skeleton (attributes, slots, CSS parts, events) but implemented with element-js on the shared style layer from the "style layer" issue in this milestone.

Loading placeholder with effect (none/sheen/pulse), shape controlled by CSS custom properties, aria-busy guidance in the readme.

Acceptance criteria

• src/components/skeleton/ with the standard files: skeleton.js (class + define()), skeleton.define.js, skeleton.style.js (composed from the shared style modules), skeleton.events.js where events exist, skeleton.stories.js, skeleton.readme.md, skeleton.changelog.md, skeleton.test.unit.js, skeleton.test.feature.js.
• Attributes reflect to properties; sizes small/medium/large and the variant vocabulary of the style layer where they apply.
• Accessible: correct roles/ARIA, keyboard operable, focus visible; axe passes in the feature test.
• Renders server-side through @webtides/element-js-ssr-renderer (catalog smoke test passes) and hydrates without a flash.
• Custom-elements manifest, catalog, all.js, and README component list updated.

Depends on

Style layer issue of this milestone.

[eddyloewen]

Depends on the style layer: #69