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

src/Support/Stylesheets.php

@@ -92,6 +92,29 @@ final class Stylesheets
         self::$cache = [];
     }
 
+    /**
+     * Every file `$files` reaches, transitively, through the same `@import` resolution `bundle()`
+     * walks — real paths, `$files` themselves included — without concatenating anything.
+     * `DesignGuard::missingStylesheets()` (plan step 41) asks this: whether an application's CSS
+     * entry's import graph already reaches a given package stylesheet, which needs the file
+     * identities `bundle()` resolves, not the CSS it produces. Shares `bundle()`'s cache (and so
+     * its freshness rule) under the same key, `$base` fixed to `null`, since only `bundle()` ever
+     * populates it.
+     *
+     * @param  list<string>  $files
+     * @return list<string>
+     */
+    public static function resolvedFiles(array $files): array
+    {
+        if ($files === []) {
+            return [];
+        }
+
+        self::bundle($files);
+
+        return array_keys(self::$cache[serialize([$files, null])]['files']);
+    }
+
     /**
      * @param  array<string, int|false>  $files  every file a cached bundle inlined, with its mtime then
      */