chore: remove the dead SSRState.fallback field and make the server state fork explicit

[Nic-Polumeyv]

First stage of #16519: the dead field and the fork semantics. No behavior change; the new module is the seam later stages merge into.

The dev context half will look different once the Vite middleware approach lands.

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 4f7854c:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/4f7854c91df3c23186a362da7965dcc02b1ca5c9

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16598

Note

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🦋 Changeset detected

Latest commit: 4f7854c

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

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@sveltejs/kit	Patch

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