fix: reduce per-request context and fallback overhead

Make Context keys lazy so requests that never call Set stop allocating on reset. Reuse stable 404 and 405 handlers and add focused benchmarks so ServeHTTP miss paths stay measurable.

context.go

@@ -97,7 +97,7 @@ func (c *Context) reset(w http.ResponseWriter, req *http.Request) {
 	}
 	c.handlers = nil
 	c.index = -1                          // 初始为 -1，`Next()` 将其设置为 0
-	c.Keys = make(map[string]any)         // 每次请求重新创建 map，避免数据污染
+	c.Keys = nil                          // 仅在首次 Set 时创建，避免每个请求都分配 map
 	c.Errors = c.Errors[:0]               // 清空 Errors 切片
 	c.queryCache = nil                    // 清空查询参数缓存
 	c.formCache = nil                     // 清空表单数据缓存