fix(MDCRenderer): keep async component identity stable across renders

[hendrikheil]

🔗 Linked issue

Ref nuxt-content/nuxt-studio#383

📚 Description

defineAsyncComponent returns a new component type on every call, and resolveComponentInstance calls it from inside the render path:

if ('setup' in _component) {
  return defineAsyncComponent(() => new Promise(resolve => resolve(_component)))
}

resolveComponentInstance is passed straight into _renderSlots as resolveComponent and invoked per node on every render, so any component reaching this branch gets a different type on each pass. Vue can't patch across a type change — it unmounts and remounts the node, and the async wrapper renders nothing for a tick while it re-resolves.

Caching the wrapper in a WeakMap keyed by the resolved component keeps the type identity stable, so repeated renders patch instead of remount. Sharing one wrapper across usages is what a plain component import already does — the wrapper holds no per-instance state.

In practice most Nuxt setups take the AsyncComponentWrapper early-return above this branch and never reach it, so this is mostly latent there; it's more likely to bite standalone MDC users. The companion fix for the path that does fire under Nuxt Content is nuxt/content#3835.

Found while tracking down nuxt-content/nuxt-studio#383, where Studio's live preview remounts the whole rendered document on every keystroke.

📝 Checklist

• I have linked an issue or discussion.
• I have added tests (none — component-identity fix with no observable output change; existing suite passes, 63/63).

[pkg-pr-new]

npm i https://pkg.pr.new/@nuxtjs/mdc@504

commit: ac1d290

[hendrikheil]

Actually, I think this may not be reachable in practice, so I'll close this for now and rely on nuxt/content#3835 for now