v0.30.0: DiscInputs carries the disc's volume label (title)

Add `DiscInputs.volume_label: Option<String>` — the disc's human title (UDF/ISO
volume identifier, else BDMV <di:name>), populated by Disc::inputs() from the
scan. Identity only, no secret, not used in any AACS derivation; lets a key
source forward the title so a key service can catalog disc_hash → title.

src/disc/mod.rs

@@ -1672,6 +1672,16 @@ impl Disc {
             // Content samples need the disc reader, which scan does not retain;
             // the caller fills these for sources that validate against ciphertext.
             samples: Vec::new(),
+            // Human title: prefer the UDF/ISO volume identifier, fall back to the
+            // BDMV display name. Identity only — a key service may catalog it.
+            volume_label: {
+                let v = self.volume_id.trim();
+                if v.is_empty() {
+                    self.meta_title.clone()
+                } else {
+                    Some(v.to_string())
+                }
+            },
         })
     }