Maximum Drawdown

The maximum drawdown of a price series is the largest peak-to-trough decline, measured as a fraction of the peak:

$$\text{MDD} = \max_{t}\; \frac{\max_{s \leq t} P_s - P_t}{\max_{s \leq t} P_s}.$$

Implement max_drawdown(prices: list[float]) -> float in a single pass. Return 0.0 for a series that never declines.