fix: make sure CSS is preserved during SSR with bindings (#15645)

fixes #15543

No test because this is a one-liner in a stable part of the code-base, also it can only happen within legacy mode when using bindings which is not getting any new updates at this point

.changeset/smooth-mugs-grin.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: make sure CSS is preserved during SSR with bindings

packages/svelte/src/internal/server/index.js

@@ -49,6 +49,7 @@ export function copy_payload({ out, css, head, uid }) {
  */
 export function assign_payload(p1, p2) {
 	p1.out = p2.out;
+	p1.css = p2.css;
 	p1.head = p2.head;
 	p1.uid = p2.uid;
 }