Extract catalogID into separate computed property for Song

For consistency with Album and improved code modularity, extract the catalogID
retrieval logic from playParameters into its own computed property on Song.

This makes the code more reusable and maintains consistency across all music
item types.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: rudrankriyam

Sources/MusadoraKit/Library/MLibrary+Song.swift

@@ -506,7 +506,19 @@ public extension Song {
   /// - Throws: An error if the song is not in library or if the request fails.
   var inFavorites: Bool {
     get async throws {
-      // Get catalog ID from play parameters
+      let catalogId = try self.catalogID
+      return try await InFavoritesParser.fetchInFavorites(for: catalogId, itemType: .songs)
+    }
+  }
+
+  /// The catalog identifier for the song.
+  ///
+  /// This property decodes the play parameters of the song to retrieve its catalog identifier.
+  ///
+  /// - Returns: The catalog ID of the song.
+  /// - Throws: `MusadoraKitError.notFound` if play parameters or catalog ID are not available.
+  var catalogID: MusicItemID {
+    get throws {
       guard let playParameters = playParameters else {
         throw MusadoraKitError.notFound(for: "playParameters")
       }
@@ -518,7 +530,7 @@ public extension Song {
         throw MusadoraKitError.notFound(for: "catalogId")
       }
 
-      return try await InFavoritesParser.fetchInFavorites(for: catalogId, itemType: .songs)
+      return catalogId
     }
   }
 }