Function cwt_fft 

pub fn cwt_fft<T>(
    signal: &Signal<T>,
    wavelet: &dyn ContinuousWavelet,
    scales: &[f64],
    sampling_freq: f64,
) -> Result<CWTResult<T>>
where
    T: Float + FromPrimitive + Send + Sync + SignalType + AddAssign + SubAssign + MulAssign + DivAssign + RemAssign,

Optimized CWT using FFT-based convolution

This implementation uses the Fast Fourier Transform for efficient convolution, which is particularly beneficial for longer signals (N > ~128 samples). The complexity is reduced from O(N²) to O(N log N) per scale.