fix(kit): ensure instrumentation file is posixified on Windows (#14993)

* fix: ensure correct instrumentation file name on Windows

* chore: add changeset file

* Apply suggestion from @teemingc

---------

Co-authored-by: Tee Ming <chewteeming01@gmail.com>

Author: BastiDood

.changeset/old-boxes-rhyme.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: posixify the instrumentation file import on Windows

packages/kit/src/core/adapt/builder.js

@@ -8,7 +8,7 @@ import { extname, resolve, join, dirname, relative } from 'node:path';
 import { pipeline } from 'node:stream';
 import { promisify } from 'node:util';
 import zlib from 'node:zlib';
-import { copy, rimraf, mkdirp } from '../../utils/filesystem.js';
+import { copy, rimraf, mkdirp, posixify } from '../../utils/filesystem.js';
 import { generate_manifest } from '../generate_manifest/index.js';
 import { get_route_segments } from '../../utils/routing.js';
 import { get_env } from '../../exports/vite/utils.js';
@@ -268,8 +268,8 @@ export function create_builder({
 				copy(`${entrypoint}.map`, `${start}.map`);
 			}
 
-			const relative_instrumentation = relative(dirname(entrypoint), instrumentation);
-			const relative_start = relative(dirname(entrypoint), start);
+			const relative_instrumentation = posixify(relative(dirname(entrypoint), instrumentation));
+			const relative_start = posixify(relative(dirname(entrypoint), start));
 
 			const facade =
 				'generateText' in module