internal/rosa: mirror response write methods

For the fs.FS implementation and static publishing.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/mbf/main.go

@@ -544,7 +544,9 @@ func main() {
 			if base, err := os.OpenRoot(cm.base); err != nil {
 				return err
 			} else {
-				h = rosa.NewMirror(msg, base.FS(), key)
+				var mux http.ServeMux
+				rosa.NewMirror(msg, base.FS(), key).Register(&mux)
+				h = &mux
 			}
 
 			server := http.Server{Addr: args[0], Handler: h}