A client wants to trade realized variance, but the market only has standard European options. Assuming $dS_t = rS_t dt + \sigma(S_t, t)S_t dW_t$, can you use Ito's Lemma to derive a portfolio that perfectly replicates $\int_0^T \sigma^2 dt$? Practically, what is the biggest risk of this hedge?

客户想交易股票的“实现方差”，但市场上只有标准欧式期权。假设股票遵循局部波动率过程 $dS_t = rS_t dt + \sigma(S_t, t)S_t dW_t$，你能利用 Ito 引理推导出一个完全复制 $\int_0^T \sigma^2 dt$ 的投资组合吗？实战中这个对冲组合最大的风险是什么？