Remote Functions: allow returning raw binary responses

Describe the problem

When dealing with binary data, I would like to avoid the serialization/deserialization and size overhead caused by passing it through devalue.

Currently, doing so requires creating a separate +server.ts endpoint and manually fetching it on the client. While that is simple enough, if such thing is feasible and makes sense, I would prefer to retain the ergonomics of remote functions while returning binary responses.

Describe the proposed solution

Provide a way for a remote function to return binary data

export const generate_pdf = query({ text: v.string() }, async ({ text }) => {
	const file = await get_file();
	const processed = await fill_with_user_input(file, text);

	return binary(processed);
});

or even avoid the binary() abstraction and just let the user handle it with a standard response:

return request.headers.get('if-none-match') === etag
	? new Response(null, {
			status: 304,
			headers: { etag }
		})
	: new Response(await processed.save(), {
			headers: {
				'content-type': 'application/pdf',
				'etag': random_hash(),
				'cache-control': 'private, no-cache'
			}
		});

And ideally consume it like this on the client:

<script lang="ts">
	import { generate_pdf } from './pdf.remote';
	import ShowPDF from './ShowPDF.svelte';
</script>

{let text = $state('')}
{let pdf = $state<Blob>()}

<input bind:value={text} />

<button onclick={async () => pdf = await generate_pdf({ text })}>
	Update PDF
</button>

<ShowPDF file={pdf} />

Alternatives considered

• Continue having +server.ts endpoint for fetching the updated file, while having the additional logic that manipulates that same file in a different handling.remote.ts

• Avoid the HTTP approach and defer this to what SvelteKit will end up with for their WebSocket implementation.

Importance

nice to have

Additional Information

Related to #16285 only if the APIs introduced there end up overlapping with this proposal. (Although my issue has more to do with same-origin responses)

A similar need for efficient binary transport was also mentioned here:
Originally posted by @garth in https://github.com/sveltejs/kit/discussions/13897#discussioncomment-14558984

Start by reviewing the remote-function entry points and the handling.remote.ts and +server.ts approaches mentioned in the issue, then compare how responses are currently serialized and consumed. Define the supported response shape and client behavior, including binary data and standard HTTP responses, and validate the design against the generate_pdf example and the related #16285 discussion.