Function weighted_average_duration 

pub fn weighted_average_duration(
    history: &ExecutionHistory,
    runner: &str,
    model: &str,
    phase_count: u8,
    phase: ExecutionPhase,
) -> Option<Duration>

Calculate weighted average duration for a specific phase.

Uses exponential weighting where recent entries are weighted higher. weight = 0.9^(age_in_days)