`page.data` mutable in legacy mode

Describe the bug

I found that the new page.data is mutable in legacy mode but not in rune mode.

Reproduction

Can't import from the new '$app/state` module in Sveltelab for unknown reasons (Kit updated). So here is the code:

//+page.server.ts
export async function load() {
    return { random: Math.random() }
}

<script lang="ts">
	import { page } from '$app/state';
</script>

{page.data.random}
<button onclick={() => page.data.random++}> Increment </button>

Logs

No response

System Info

N/A

Severity

annoyance

Additional Information

No response

Start with the provided +page.server.ts load function and the page.data example, comparing behavior in legacy and rune modes. Trace the page state handling that permits the legacy mutation and verify that the reproduction no longer allows page.data.random++ to mutate the loaded data.