Function compute_custom_waves 

pub(super) fn compute_custom_waves(
    slot: &mut PresetSlot,
    left: &[f32],
    right: &[f32],
    spectrum_bins: &[f32],
) -> (Vec<CustomWaveVertex>, Vec<CustomWaveBatch>)

Run every enabled wavecode_N block in slot’s preset and return the per-frame vertex stream + batches. Returns empty Vecs when no preset is loaded or no waves are enabled — feed those through to the renderer’s update_custom_waves to reset its buffers.