fix: route result output through Config.Output, not os.Stdout

mark.Run and ProcessFile were writing directly to os.Stdout via
fmt.Println, which is a surprising side-effect for library callers.

Add Config.Output io.Writer for callers to provide their own sink.
When nil the helper falls back to io.Discard, so library embedders
that do not set Output receive no implicit stdout writes. The CLI
layer sets Output: os.Stdout to preserve existing behaviour.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

util/cli.go

@@ -83,6 +83,8 @@ func RunMark(ctx context.Context, cmd *cli.Command) error {
 		Features:        cmd.StringSlice("features"),
 		ImageAlign:      cmd.String("image-align"),
 		IncludePath:     cmd.String("include-path"),
+
+		Output: os.Stdout,
 	}
 
 	return mark.Run(config)