feat: support parent parameter in generateMetadata()

[james-elicx]

Summary

Next.js's generateMetadata() accepts a second parent parameter of type Promise<ResolvedMetadata>, which resolves to the accumulated metadata from ancestor segments. This allows patterns like:

export async function generateMetadata({ params }, parent) {
  const previousImages = (await parent).openGraph?.images || []
  return { openGraph: { images: ['/new-image.jpg', ...previousImages] } }
}

Next.js uses an eager-execution-with-serial-resolution approach: all generateMetadata() calls are kicked off concurrently, but the parent promise for each segment resolves serially from root to leaf.

Current behavior

vinext's resolveModuleMetadata() does not pass a parent argument. Apps using await parent in generateMetadata() receive undefined and will likely throw or produce incorrect metadata.

Expected behavior

generateMetadata() should receive a parent promise that resolves to the merged metadata from all ancestor segments, matching Next.js behavior.

References

• Next.js docs: generateMetadata — parent parameter
• Flagged during review of perf: parallelise metadata and viewport resolution #372