refactor!: clean up stdio interface

with an interface no less!

src/ChildStderr.php

@@ -4,7 +4,15 @@ declare(strict_types=1);
 
 namespace Nih\CommandBuilder;
 
-final class ChildStderr
+use Override;
+
+final class ChildStderr implements StdioInterface
 {
     use StreamReadable;
+
+    #[Override]
+    public function getDescriptionSpec(int $fd): mixed
+    {
+        return $this->stream;
+    }
 }