* fix(metadata): match Next dynamic route semantics
Metadata routes diverged from Next.js in several observable cases: robots.txt emitted Sitemap before Host, dynamic metadata Response results could miss framework cache headers, TSX sitemap files were not discovered, and rendered metadata URLs treated canonical, manifest, and file-based social image routes as the same URL class.
The implementation now follows Next's metadata route extension, cache, robots, and URL resolution rules while keeping file-based social image route markers non-enumerable so resolved metadata shape does not leak internal state.
* fix(metadata): gate social image preview fallback
File-based social image routes used the preview deployment URL whenever VERCEL_URL was present outside development. That let deployment URLs override an explicit metadataBase in non-preview production environments.
Gate preview URL fallback on VERCEL_ENV=preview, add coverage for metadataBase precedence and production fallback URLs, and keep static metadata image cache headers stable in development.
Nathan Nguyen