fix: ensure component root effect updates occur first

[trueadm]

A better fix than #15437. We should prioritize updates to component root effects before other root effect types – this ensures teardown happens before we process other root effects.

Fixes #15426.

[changeset-bot]

🦋 Changeset detected

Latest commit: 9008723

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

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15439

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15439-svelte.vercel.app/

[Rich-Harris]

It's possible for a similar problem to occur without components being involved — the inner effect runs before the outer one:

let obj = $state({ count: 0 });
  
$effect.root(() => {
  let teardown;

  $effect.pre(() => {
    if (obj) {
      teardown ??= $effect.root(() => {
        $effect.pre(() => {
          console.log(obj.count);
        });
      });
    } else {
      teardown?.();
      teardown = null;
    }
  });
});

This is obviously rather contrived but it makes me wonder if dividing effect roots into component and non-component roots is the right approach, or if we should process roots-with-parents in tree order instead. Opened #15446 for comparison