fix: ensure hydration walks all nodes

We've marked several methods used for walking the DOM with a `__NO_SIDE_EFFECTS__` comment. That was good historically, because we didn't need those kept around if its results were unused, but since the hydration changes in #12335 this actually introduces a bug: Because that PR now relies on the hydration nodes being correct due to walking the DOM, tree-shaking unused variables/calls results in the walk being incorrect, leading to bugs

Fixes #12422

Author: dummdidumm

.changeset/giant-jars-applaud.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure hydration walks all nodes

packages/svelte/src/internal/client/dom/operations.js

@@ -51,11 +51,11 @@ export function empty() {
 }
 
 /**
+ * Don't mark this as side-effect-free, hydration needs to walk all nodes
  * @template {Node} N
  * @param {N} node
  * @returns {Node | null}
  */
-/*#__NO_SIDE_EFFECTS__*/
 export function child(node) {
 	if (!hydrating) {
 		return node.firstChild;
@@ -73,11 +73,11 @@ export function child(node) {
 }
 
 /**
+ * Don't mark this as side-effect-free, hydration needs to walk all nodes
  * @param {DocumentFragment | TemplateNode[]} fragment
  * @param {boolean} is_text
  * @returns {Node | null}
  */
-/*#__NO_SIDE_EFFECTS__*/
 export function first_child(fragment, is_text) {
 	if (!hydrating) {
 		// when not hydrating, `fragment` is a `DocumentFragment` (the result of calling `open_frag`)
@@ -103,12 +103,12 @@ export function first_child(fragment, is_text) {
 }
 
 /**
+ * Don't mark this as side-effect-free, hydration needs to walk all nodes
  * @template {Node} N
  * @param {N} node
  * @param {boolean} is_text
  * @returns {Node | null}
  */
-/*#__NO_SIDE_EFFECTS__*/
 export function sibling(node, is_text = false) {
 	if (!hydrating) {
 		return /** @type {TemplateNode} */ (node.nextSibling);

packages/svelte/src/internal/client/dom/template.js

@@ -207,7 +207,9 @@ function run_scripts(node) {
 	}
 }
 
-/*#__NO_SIDE_EFFECTS__*/
+/**
+ * Don't mark this as side-effect-free, hydration needs to walk all nodes
+ */
 export function text() {
 	if (!hydrating) {
 		var t = empty();