Sturdy Rust playground refactor

Fixed-window moving average with constant-time updates.

use std::collections::VecDeque;

pub struct MovingAverage {
    window: VecDeque<f64>,
    capacity: usize,
    sum: f64,
}

impl MovingAverage {
    pub fn new(capacity: usize) -> Self {
        Self { window: VecDeque::with_capacity(capacity), capacity, sum: 0.0 }
    }

    pub fn push(&mut self, value: f64) -> f64 {
        if self.window.len() == self.capacity {
            if let Some(old) = self.window.pop_front() {
                self.sum -= old;
            }
        }
        self.window.push_back(value);
        self.sum += value;
        self.sum / self.window.len() as f64
    }
}