fix: freeze deriveds once their containing effects are destroyed

[Rich-Harris]

Per #17862 (comment), this freezes the value of a derived if it was created inside a parent effect that is now destroyed. This prevents the sort of bug where a derived reads foo.bar even though foo is now undefined.

If the derived is dirty, a warning will be printed.

This PR also gets rid of some weirdness around derived.parent — it can only ever be an Effect | null, and there's no need for get_derived_parent_effect.

Blocked on sveltejs/kit#15533 and a follow-up that switches remote functions to use $effect.root (since this effectively undoes #17171), hence draft.

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Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: a549d39

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

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

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[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@17921

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-17921-svelte.vercel.app/

[Rich-Harris]

/autofix

[Rich-Harris]

/autofix

[codeCraft-Ritik]

Nice implementation