fix: remove nodes in boundary when work is pending and HMR is active (#17932)

Fixes
https://github.com/sveltejs/svelte/issues/17918#issuecomment-4054067024.

The issue here was that in `boundary.#render`, if `this.#pending_count >
0` we yoink the content out of the DOM so we can replace it with the
`pending` fragment. This works by taking everything from
`effect.nodes.start` to `effect.nodes.end` and putting it in a
`DocumentFragment`.

With HMR, that doesn't work, because the effect with the nodes is buried
inside the HMR effect. This fixes it.

Draft because I'd like to try this out in a few more places before
merging.

.changeset/tangy-women-shout.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: remove nodes in boundary when work is pending and HMR is active

packages/svelte/src/internal/client/dev/hmr.js

@@ -6,6 +6,8 @@ import { block, branch, destroy_effect } from '../reactivity/effects.js';
 import { set, source } from '../reactivity/sources.js';
 import { set_should_intro } from '../render.js';
 import { get } from '../runtime.js';
+import { assign_nodes } from '../dom/template.js';
+import { create_comment } from '../dom/operations.js';
 
 /**
  * @template {(anchor: Comment, props: any) => any} Component
@@ -27,6 +29,13 @@ export function hmr(fn) {
 
 		let ran = false;
 
+		// Surround the wrapped effects with comments and assign the nodes
+		// on the wrapping effects so the parent can properly do DOM operations.
+		let start = create_comment();
+		let end = create_comment();
+
+		anchor.before(start);
+
 		block(() => {
 			if (component === (component = get(current))) {
 				return;
@@ -61,6 +70,10 @@ export function hmr(fn) {
 			anchor = hydrate_node;
 		}
 
+		anchor.before(end);
+
+		assign_nodes(start, end);
+
 		return instance;
 	}