fix: account for server-emitted assets when prerendering (#9349)

fixes #9146

.changeset/polite-buckets-fold.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: account for server-emitted assets when prerenering

packages/adapter-static/test/apps/prerendered/src/routes/server-emitted-asset/+page.server.js

@@ -0,0 +1,7 @@
+import src from './image.jpg';
+
+export function load() {
+	return {
+		src
+	};
+}

packages/adapter-static/test/apps/prerendered/src/routes/server-emitted-asset/+page.svelte

@@ -0,0 +1,5 @@
+<script>
+	export let data;
+</script>
+
+<img src={data.src} alt="birb" />

packages/kit/src/core/postbuild/prerender.js

@@ -1,4 +1,4 @@
-import { readFileSync, writeFileSync } from 'node:fs';
+import { existsSync, readFileSync, writeFileSync } from 'node:fs';
 import { dirname, join } from 'node:path';
 import { pathToFileURL } from 'node:url';
 import { installPolyfills } from '../../exports/node/polyfills.js';
@@ -144,6 +144,13 @@ async function prerender({ out, manifest_path, metadata, verbose, env }) {
 	}
 
 	const files = new Set(walk(`${out}/client`).map(posixify));
+
+	const immutable = `${config.appDir}/immutable`;
+	if (existsSync(`${out}/server/${immutable}`)) {
+		for (const file of walk(`${out}/server/${immutable}`)) {
+			files.add(posixify(`${config.appDir}/immutable/${file}`));
+		}
+	}
 	const seen = new Set();
 	const written = new Set();