fix(06-14): actually reuse built middleware factories per name:param

The built cache added in 06-12 was declared but never consulted, so every
route re-invoked the factory on BuildRouter and again on compile.

surf/router.go

@@ -398,7 +398,12 @@ func (r *Router) wrap(rt route) (http.Handler, error) {
 						return nil, fmt.Errorf("surf: plugin %q: %w", rt.pluginID, err)
 					}
 				}
-				h = factory.fn(param)(h)
+				mw, ok := r.built[name]
+				if !ok {
+					mw = factory.fn(param)
+					r.built[name] = mw
+				}
+				h = mw(h)
 				continue
 			}
 		}