Function verify_shadow_decoder_side 

pub fn verify_shadow_decoder_side(
    img: &JpegImage,
    state: &ShadowState,
    passphrase: &str,
    stego_y_positions_sorted: &[CoeffPos],
) -> Result<(), StegoError>

Verify a shadow can be decoded from the decoder’s perspective.

Uses stego-image UNIWARD costs (not encoder’s cover costs) to select positions, then checks RS decode + AES-GCM. This catches cost-pool boundary disagreements that the encoder-side verify misses.

stego_y_positions_sorted must be sorted by cost from the STEGO image.