add test sample

packages/svelte/tests/runtime-runes/samples/fork-derived-class-instance/Child.svelte

@@ -0,0 +1,10 @@
+<script>
+	class Y {
+		foo = $state(0);
+	}
+
+	let y = $derived(new Y());
+</script>
+
+<button onclick={() => y.foo++}>click</button>
+<p>{y.foo}</p>

packages/svelte/tests/runtime-runes/samples/fork-derived-class-instance/_config.js

@@ -0,0 +1,24 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip_no_async: true,
+	async test({ assert, target }) {
+		const forkButton = target.querySelector('button');
+
+		flushSync(() => {
+			forkButton?.click();
+		});
+
+		const [, clickButton] = target.querySelectorAll('button');
+		const p = target.querySelector('p');
+
+		assert.equal(p?.textContent, '0');
+
+		flushSync(() => {
+			clickButton?.click();
+		});
+
+		assert.equal(p?.textContent, '1');
+	}
+});

packages/svelte/tests/runtime-runes/samples/fork-derived-class-instance/main.svelte

@@ -0,0 +1,17 @@
+<script>
+	import { fork } from 'svelte';
+	import Child from './Child.svelte';
+
+	let x = $state(false);
+</script>
+
+<button onclick={() => {
+	const f = fork(() => {
+		x = true;
+	});
+	f.commit();
+}}>fork</button>
+
+{#if x}
+	<Child />
+{/if}