unlock: finalize Unlocker 3-capability contract

Rename the trait to a generic, drive-neutral capability contract so future
unlockers don't conform to LibreDrive specifics:

  - unlock(...)   -> unlock_drive(...)        (the one required capability)
  - read_vid(...) -> read_volume_id(...)      (no-op default)
  - add set_max_read_speed(...)               (no-op default)

The trait doc now states the contract in one place: unlockers are optional
drive-capability providers; the AACS layer is the always-present baseline and
falls back to the full cert handshake when no unlocker matches. Implement only
the capabilities your drive supports.

Registry: route_unlock now calls unlock_drive; unlocker_read_vid renamed to
unlocker_read_volume_id; add unlocker_set_max_read_speed (mirrors route_unlock
resolution, first matching unlocker, no-op if none match). drive::init calls
it on a matched drive in the post-unlock path; a speed-set failure is logged
and does not fail the rip. encrypt.rs handshake updated to the new VID helper.

Tests updated for the renames; added a set_max_read_speed routing test
(match invokes, no-match is a safe no-op).

src/disc/encrypt.rs

@@ -76,7 +76,7 @@ impl Disc {
         // DriveId first releases the immutable borrow before we hand the
         // mutable transport to the registry.
         let drive_id = session.drive_id.clone();
-        match crate::unlock::unlocker_read_vid(session.scsi_mut(), &drive_id) {
+        match crate::unlock::unlocker_read_volume_id(session.scsi_mut(), &drive_id) {
             Ok(Some(volume_id)) => {
                 tracing::debug!(
                     target: "freemkv::disc",