P2b: offline WAV render of the arrangement

crates/zhila-protocol/src/lib.rs

@@ -218,6 +218,7 @@ pub enum Command {
         from: u8,
         to: u8,
     },
+    Render,
     Undo,
     Redo,
     /// Journal-only: a core was imported through the binary path.
@@ -277,6 +278,10 @@ pub enum Event {
         sections: Vec<Pattern>,
         active: u8,
     },
+    Rendered {
+        path: String,
+        duration_sec: f32,
+    },
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]