What problem does this feature solve?

Vite uses shouldTransformCachedModule hook for build watch mode. shouldTransformCachedModule is not implemented in Rolldown. I'm not sure how Rolldown handles the changes in watch mode, so this might not be needed or a different way should be explored.

What does the proposed API look like?

https://rollupjs.org/plugin-development/#shouldtransformcachedmodule