Add keys() to IOStream trait — streams know their own decrypt keys

- IOStream::keys() default returns DecryptKeys::None
- IsoStream and DiscStream override with real AACS/CSS keys
- Pipeline calls input.keys() instead of separate scan_keys()
- Streams are self-contained: read bytes + provide keys

src/mux/iso.rs

@@ -231,13 +231,15 @@ impl IOStream for IsoStream {
         Ok(())
     }
     fn total_bytes(&self) -> Option<u64> {
-        // Read mode: size is known from disc scan
         if self.reader.is_some() {
             Some(self.disc_title.size_bytes)
         } else {
             None
         }
     }
+    fn keys(&self) -> DecryptKeys {
+        self.decrypt_keys.clone()
+    }
 }
 
 impl Read for IsoStream {