Hash API routes for static builds

Describe the problem

SvelteKit always fetches a new copy of a route's data when navigating to a new page or using sveltekit:prefetch. If the website was built using the static adaptor, we know the data is static and there is no need to refetch and check for updates.

If you visit https://svelte-demo.jamcart.io/, use dev tools to throttle the connection, and use the browser's back/forward buttons to navigate, then that delay becomes noticeable. This is not an issue for Next.js (https://react-demo.jamcart.io/), Nuxt.js (https://vue-demo.jamcart.io/), or even just static html (https://astro-demo.jamcart.io/).

Describe the proposed solution

Adopt Next/Nuxt's approach. Instead of writing files out to /api/my-endpoint.json, it could be instead rewritten to /_app/{ BUILD HASH OR TIMESTAMP }/api/my-endpoint.json. Then, developers can simply apply Cache-Control: public, immutable to everything inside of /_app.

Alternatives considered

For a static built, SvelteKit could just keep previous requests in memory and not force a refetch.

Importance

nice to have

Additional Information

No response

Start by tracing how the static adapter writes API route data and how SvelteKit fetches route data during navigation and prefetching. Compare the proposed hashed /_app output with the current static route paths and the caching behavior described in the issue. Done means static builds avoid unnecessary refetches while generated routes remain reachable and cacheable.