Function shadow_extract 

pub fn shadow_extract(
    img: &JpegImage,
    all_y_positions_sorted: &[CoeffPos],
    passphrase: &str,
) -> Result<PayloadData, StegoError>

Full shadow decode pipeline (headerless brute-force).

Brute-forces (fraction, parity, fdl) combinations. For each cost fraction, selects positions from the cheapest pool, then tries all parity/fdl combos. AES-256-GCM-SIV authentication validates correct parameters.

all_y_positions_sorted must be sorted by cost (cheapest first).