feat: Add logic_mut() to Node and fix playlist registration in FlacCacheSink

- Add Node::logic_mut() method to allow post-construction configuration
  of node logic before run() is called
- Fix FlacCacheSink to properly transfer playlist_handle_pending to
  the inner logic using logic_mut()
- Resolves FIXME at flac_cache_sink.rs:656 about missing logic_mut()

This enables the playlist registration mechanism to work correctly:
1. User calls register_playlist() on FlacCacheSink
2. Handle is stored in playlist_handle_pending
3. During run(), handle is transferred to FlacCacheSinkLogic
4. Tracks are automatically added to playlist after caching

pmoaudio/src/pipeline.rs

@@ -482,6 +482,15 @@ impl<L: NodeLogic> Node<L> {
     pub fn logic(&self) -> &L {
         &self.logic
     }
+
+    /// Retourne une référence mutable vers la logique métier du nœud
+    ///
+    /// Permet de configurer la logique après construction mais avant run().
+    /// Utile pour définir des options qui ne peuvent pas être connues
+    /// au moment de la construction du nœud.
+    pub fn logic_mut(&mut self) -> &mut L {
+        &mut self.logic
+    }
 }
 
 #[async_trait::async_trait]