Add Stylesheets::resolvedFiles() for the design guard to reuse

Plan step 41 (ii): the design guard needs the set of files an application's
CSS entry pulls in, transitively, to check whether a package tag's
stylesheet is among them — reusing bundle()'s own import resolution and
cache rather than re-implementing @import parsing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

tests/Feature/StylesheetsBundleTest.php

@@ -82,6 +82,17 @@ it('bundles all.css with every import resolved, foundation first, every componen
         ->and($foundationPosition)->toBeLessThan($earliestOtherPosition);
 });
 
+it('resolves the files an entry imports, transitively, without bundling any content', function () {
+    $files = Stylesheets::resolvedFiles([stylesheetsBundlePath('components/split-button.css')]);
+
+    expect($files)->toContain(
+        stylesheetsBundlePath('components/split-button.css'),
+        stylesheetsBundlePath('components/button.css'),
+        stylesheetsBundlePath('components/menu.css'),
+        stylesheetsBundlePath('components/icon.css'),
+    );
+});
+
 it('keeps a file two others import once, at its first position', function () {
     $dir = sys_get_temp_dir().'/livewire-material-bundle-'.Str::random(8);
     File::makeDirectory($dir);