Make unlock non-fatal in open_title — BD discs work without it

BD drives reject the MediaTek unlock command (sense 0x05).
Unlock is only needed for UHD raw access. Standard BD reads
work with standard READ(10) without vendor unlock.

src/disc.rs

@@ -793,9 +793,9 @@ impl Disc {
             detail: format!("title index {} out of range (have {})", title_idx, self.titles.len()),
         })?;
 
-        // Ensure drive is unlocked
+        // Try to unlock for raw reads. Non-fatal — BD discs work without unlock.
         if !session.is_unlocked() {
-            session.unlock()?;
+            let _ = session.unlock();
         }
 
         // Set max read speed — nothing else. No calibration, no probes.