fix: don't depend on deriveds created inside the current reaction

[Rich-Harris]

Follow-up to #15553 which fixes this case.

Unfortunately it changes the behaviour of effects in a way that could be considered breaking — in a case like this, where a derived that's local to an effect depends on state outside the effect...

let count = $state(0);

$effect(() => {
  let double = $derived(count * 2)

  console.log('init ' + double);

  return function() {
    console.log('cleanup ' + double);
    console.log(this);
  };
});

...today the effect will re-run when count changes, but in this PR it won't. I'm not sure there's a way to have that behaviour and have effects not depend on deriveds that depend entirely on local state. I think this is the right trade-off, but I'd love to hear from other maintainers.

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Playground

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Preview: https://svelte-dev-git-preview-svelte-15564-svelte.vercel.app/

[dummdidumm]

If we don't want effects to rerun when state they create changes, then by extension we also don't want that for deriveds, regardless what sources they have. I also don't know in what situation you would create a derived inside an effect and want it to re-run when said derived changes.
Having said all that, these nuances make it increasingly hard to explain when effects rerun - we need to find a good mental model for that and put it down in the docs.

[trueadm]

I think this is a sensible change, but it's also maybe a breaking change. I guess we can release it and address if that is the case.

[maxnowack]

Unfortunately this breaks the svelte integration of SignalDB 😕
See maxnowack/signaldb#1536

Some more information about the integration: https://signaldb.js.org/guides/svelte/

cc @ykrods