fix: ensure head effects are kept in the effect tree (#17746)

Fixes #17726

The problem was that the head effect had no "keep me around"-flag, which
it needs because its children are not guaranteed to be present
immediately - as shown in the related issue, where the child effect is
only created once async work has completed.

### Before submitting the PR, please make sure you do the following

- [x] It's really useful if your PR references an issue where it is
discussed ahead of time. In many cases, features are absent for a
reason. For large changes, please create an RFC:
https://github.com/sveltejs/rfcs
- [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
- [x] This message body should clearly illustrate what problems it
solves.
- [x] Ideally, include a test that fails without this PR but passes with
it.
- [x] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).

### Tests and linting

- [x] Run the tests with `pnpm test` and lint the project with `pnpm
lint`

.changeset/old-crabs-dance.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure head effects are kept in the effect tree

packages/svelte/src/internal/client/dom/blocks/svelte-head.js

@@ -2,7 +2,7 @@
 import { hydrate_node, hydrating, set_hydrate_node, set_hydrating } from '../hydration.js';
 import { create_text, get_first_child, get_next_sibling } from '../operations.js';
 import { block } from '../../reactivity/effects.js';
-import { COMMENT_NODE, HEAD_EFFECT } from '#client/constants';
+import { COMMENT_NODE, EFFECT_PRESERVED, HEAD_EFFECT } from '#client/constants';
 
 /**
  * @param {string} hash
@@ -49,7 +49,9 @@ export function head(hash, render_fn) {
 	}
 
 	try {
-		block(() => render_fn(anchor), HEAD_EFFECT);
+		// normally a branch is the child of a block and would have the EFFECT_PRESERVED flag,
+		// but since head blocks don't necessarily only have direct branch children we add it on the block itself
+		block(() => render_fn(anchor), HEAD_EFFECT | EFFECT_PRESERVED);
 	} finally {
 		if (was_hydrating) {
 			set_hydrating(true);

packages/svelte/tests/runtime-runes/samples/async-derived-title-update/_config.js

@@ -0,0 +1,12 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+
+		const p = target.querySelector('p');
+		assert.equal(p?.innerHTML, 'hello');
+		assert.equal(window.document.title, 'hello');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-derived-title-update/main.svelte

@@ -0,0 +1,10 @@
+<script>
+	let promise = Promise.resolve('hello');
+	const value = $derived(await promise);
+</script>
+
+<svelte:head>
+	<title>{value}</title>
+</svelte:head>
+
+<p>{value}</p>