Function stft 

pub fn stft(
    signal: &[f64],
    window_size: usize,
    hop_size: usize,
    window_fn: &[f64],
) -> Vec<Vec<Complex>>

Compute the Short-Time Fourier Transform of a real signal.

Arguments

• signal – real-valued input signal.
• window_size – number of samples per window (power of two recommended).
• hop_size – step between successive windows (≤ window_size).
• window_fn – window function applied to each frame (e.g., Hann window).

Returns a Vec<VecComplex> where each inner Vec is the FFT of one frame. The outer dimension is time (frames), inner is frequency.