New src/session.rs providing DiscSession, DeviceTarget, and KeySpec,
re-exported from lib.rs. DiscSession::open runs the drive open +
advisory wait_ready/init/probe_disc bring-up (owning the Drive by
value); identify()/scan() are split so consumers keep their own
UI/TMDB sequencing. KeySpec carries consumer-built credentials +
key_sources which scan() forwards into ScanOptions without clobbering
caller-set fields (the library derives no certs and reads no keydb).
Unit tests cover the KeySpec->ScanOptions forwarding.
