Use of runes in SvelteKit load functions

[bertie-sektorcert]

Describe the bug

It doesn't seem like there's a way to use the svelte runes within the +page.ts, +layout.ts, +page.server.ts and +layout.server.ts, and it doesn't seem like there's any documentation. This seem like a major limiting factor in implementing reactivity using the new methods. Attempting to do so gives the following error:

Svelte error: rune_outside_svelte The '$state' rune is only available inside '.svelte' and '.svelte.js/ts' files

Attempting to rename the load files to something like +page.svelte.ts gives the following error:

Files prefixed with + are reserved (saw /+page.svelte.ts)

Reproduction

import type { PageLoad } from './$types';

export const load: PageLoad = () => {
    let x = $state(None)

    return {
        x
    };
};

Logs

No response

System Info

System:
    OS: macOS 15.2
    CPU: (12) arm64 Apple M3 Pro
    Memory: 1.20 GB / 36.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 23.4.0 - ~/web/node_modules/.bin/node
    npm: 10.9.0 - /opt/homebrew/bin/npm
  Browsers:
    Safari: 18.2
  npmPackages:
    @sveltejs/adapter-auto: ^3.0.0 => 3.3.1
    @sveltejs/kit: ^2.0.0 => 2.8.2
    @sveltejs/vite-plugin-svelte: ^4.0.0 => 4.0.1
    svelte: ^5.0.0 => 5.2.7
    vite: ^5.0.3 => 5.4.11

Severity

annoyance

Additional Information

No response

[kran6a]

You can already use svelte/reactivity classes there. Arrays, primitives and objects cannot be used as of now but Map, Set and Date can be.
I use this to handle live data updates via SSE and reactivity works as expected. Note that I use adapter-static without prerendering nor SSR enabled.