Function power_memory 

pub fn power_memory(n: usize) -> usize

Power-law growing memory p_n = floor(n^0.4), clamped to [2, n/4] — the Fine–Hwang-style window. Satisfies p_n²/n → 0 (Hwang) but not p_n³/n → 0 (Berk 1974 / Lewis–Reinsel 1985 uniform results). It was the package default until the benchmark showed the poly-log window dominating it (notably on seasonal AR, where n^0.4 < 12 until n ≈ 800); the default is now polylog_memory(n, 2.0) via crate::Statistic::default_memory. Kept as an option and for the benchmark.