fix: ensure effects destroy owned deriveds upon teardown

[trueadm]

Deriveds now cleanup accordingly when the effect that they were created in gets destroyed. This should resolve a memory leak with deriveds that created in branch/root effects – and thus were previously never cleaned up as these effects don't know of the deriveds in the first place. This essentially aims to solve this memory leak:

<!-- main.svelte -->
<script>
    import Foo from './Foo.svelte';

    let count = $state(0);
    let toggle = $state(0);
</script>

{#if toggle}
  {@const double = count * 2}
  <Foo double={double} />
{/if}

<button onclick={() => toggle = !toggle}>toggle</button>
<button onclick={() => count++}>increment</button>

<!-- Foo.svelte -->
<script>
    const { double } = $props();
    // read the prop, now it leaks when we toggle and hide this component
    double;
</script>

Above the double derived leaks, with this PR we now destroy that derived when the {if} block toggles. Previously, we didn't destroy it, meaning it was permanently retained forever – taking a lot of DOM and effects with it. This is because we read the double inside the component body of Foo.svelte which is a branch effect – not a tracking effect. The read causes a connection to be made with count and this connection is never disconnected upon the effects being destroyed as the effects have no knowledge of the derived.

[changeset-bot]

🦋 Changeset detected

Latest commit: cd632e5

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

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[trueadm]

Looking at the benchmarks, this doesn't seem to have any real impact on perf:

This branch:

Main:

[paoloricciuti]

Mmm this could also in some way be related to #13586 ...could you take a look at that too?