ld: drive-prep guard (fire only at kind==Unknown) + firmware_unlocker_name

Restore ld's drive-prep match guard so the firmware unlocker fires exactly once
(at drive-prep, kind==Unknown) and never re-fires during the content-keyed
Aacs/Css dispatch — the consumer (libfreemkv) dispatches at three points
(init/cert/css) with the appropriate kind. Add firmware_unlocker_name(drive_id)
for "is this drive supported?" drive-info display without unlocking. 89 tests.

src/lib.rs

@@ -114,6 +114,13 @@ pub trait Unlocker: Send + Sync {
     ) -> std::result::Result<Unlocked, UnlockError>;
 }
 
+/// Name of the firmware unlocker that supports this drive (for drive-info "is
+/// this drive supported?" display), or `None`. A pure profile lookup — does NOT
+/// touch the drive or unlock anything.
+pub fn firmware_unlocker_name(drive_id: &DriveId) -> Option<&'static str> {
+    ld::firmware_name(drive_id)
+}
+
 /// Every unlocker, in dispatch order (firmware → cert → css). This is the ONLY
 /// place an unlocker is named. Remove one = delete its line here + its module
 /// dir; the consumer never changes.