net/url: Quadratic Complexity in resolvePath During URL Resolution

Go’s net/url package resolved relative paths containing .. segments by repeatedly converting to and rewriting a string buffer at each step, giving the algorithm quadratic time complexity and high memory overhead for pathological inputs. A URL or request path with many .. segments could make URL.Parse and URL.ResolveReference consume disproportionate CPU and memory. The fix reworks resolvePath to use index-based backtracking over a byte buffer instead.