Function decrypt_sectors 

pub fn decrypt_sectors(
    buf: &mut [u8],
    keys: &DecryptKeys,
    unit_key_idx: usize,
) -> Result<(), Error>

Decrypt a buffer of sectors in-place.

For AACS: processes in 6144-byte aligned units (3 sectors). For CSS: processes per 2048-byte sector. For None: no-op.

unit_key_idx selects which AACS unit key to use (0 for most discs).

Returns Err if decryption was expected but keys are missing or invalid. Never produces silently corrupted output.