fix: clone server data before running universal load on server

[teemingc]

fixes #12236

A refactor attempt to serialize the server data before it's passed to the universal load functions.

I'm sure I haven't implemented this correctly:

1. I don't think it's correct to await all the server loads, to serialize the server data, and only then run the universal load functions (might slow down when they start and finish).
2. I also don't like the code duplication shared between rendering the page and error (and whatever needs to serialise server code in the future).

Maybe someone can take a better shot at this.

EDIT: decided on shallow cloning the server data (see https://github.com/sveltejs/kit/pull/12235#issuecomment-2415821795)

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[teemingc]

I think the simplest fix for this is to shallow clone the object because:

1. serialising the server data earlier requires awaiting all the server loads and it also messes up the timing of the chunk streaming (causes one of the tests to fail).
2. deep cloning the server data causes errors if there's a promise that's returned by the server load.

[dummdidumm]

Thanks for the exploration, but I'm closing this (for now, we can always reopen later) for the reasons given in https://github.com/sveltejs/kit/issues/12236#issuecomment-2595104925