Function select_smooth_frames 

pub fn select_smooth_frames(
    cam_dir: &Path,
    sel: &SelectTuning,
) -> Result<Vec<PathBuf>, String>

Pick the parked frame from each layer’s burst in a smooth cam dir, returning the chosen full-res JPEG paths in layer order. Decodes the whole dir to tiny gray with ONE ffmpeg, groups by layer, and runs the pure select_park_frame. Layers whose park fell outside the burst are skipped (a gap beats a head-over-print frame).