Restructure: scsi/ and aacs/ as module directories

- scsi.rs → scsi/mod.rs (trait, constants, CDB builders) + scsi/linux.rs (SG_IO)
 Ready for scsi/macos.rs and scsi/windows.rs when needed.
- aacs.rs + aacs_handshake.rs → aacs/mod.rs (keys, decrypt) + aacs/handshake.rs (SCSI auth)
- All internal refs use super:: within module, crate:: across modules
- Zero warnings, 31 tests passing

src/lib.rs

@@ -79,7 +79,6 @@ pub mod clpi;
 pub mod disc;
 pub mod jar;
 pub mod aacs;
-pub mod aacs_handshake;
 
 pub use error::{Error, Result};
 pub use drive::DriveSession;