* refactor(app-router): delegate RSC preload hint normalization
The App Router RSC entry currently owns the React Flight preload hint stream transform inline. That keeps byte-stream buffering logic inside generated code, so the behavior is only indirectly covered by generated-entry snapshots.
Move the stream normalization into a typed server helper and keep the generated RSC entry as a thin wrapper around renderToReadableStream. Add direct coverage for complete lines, split chunks, and final unterminated Flight hint lines.
* test(app-router): cover mixed Flight preload hints
Nathan Nguyen