EWMA Volatility Filter

The exponentially weighted moving-average (EWMA) variance estimator updates

$$v_t = \lambda\,v_{t-1} + (1 - \lambda)\,r_t^2 ,$$

starting from an initial variance $v_0$.

Implement ewma_variance(returns: list[float], lam: float, v0: float) -> list[float] returning $[v_1, \dots, v_n]$.