fix: serialize server load data before passing to universal load alternative

[PatrickG]

Alternative to #14268
Fixes #14291

CC @Rich-Harris

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-14298-svelte.vercel.app/

[changeset-bot]

🦋 Changeset detected

Latest commit: f3d0384

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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@sveltejs/kit	Patch

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[PatrickG]

Finally all green.