Function retime_audio 

fn retime_audio(
    full: &[f32],
    segments: &[TimelineSegment],
    source_fps: u32,
    sample_rate: u32,
    channels: usize,
) -> Vec<f32>

Retime the source audio to the edited timeline (ED.21).

full is the source’s interleaved F32LE audio (channels samples per sample-frame). For each timeline segment, the source sample-frame range [source_start, source_end) — mapped from source video frames via source_fps and sample_rate — is resampled to the segment’s project duration: a timescale of 2.0 emits half as many sample-frames (sped up), 0.5 twice as many (slowed). Linear interpolation between adjacent sample-frames keeps it click-free; the segments are then concatenated, so the result matches the retimed video frame-for-frame.

v1 ships speed-with-pitch by design: a sped-up segment rises in pitch, a slowed one drops — the industry default for editor speed ramps (Premiere / FCP / Resolve / Loom all default to it). Pitch-preserving retime (time-stretch) is a tracked enhancement, ISS-18: it slots in behind this exact pure (samples, segments, fps, rate, channels) -> Vec<f32> contract, so callers and tests don’t change when it lands. Pure — no gst, exhaustively testable.