Function make_loop_buffer 

pub fn make_loop_buffer(
    samples: &[f32],
    sr: u32,
    start_secs: f32,
    end_secs: Option<f32>,
    crossfade_secs: f32,
) -> Vec<f32>

Extract the loop region [start_secs, end_secs) and equal-power crossfade its last crossfade_secs onto its head, returning a buffer that repeats seamlessly. The output length is the region minus the crossfade.

Overlap-loop: with region r of length L and crossfade x, the body is r[0..L-x] with its first x samples replaced by a sin/cos blend of the head (r[i], fading in) and the tail (r[L-x+i], fading out). The wrap out[last] → out[0] then lands on adjacent original samples, so there is no discontinuity.