feat: Add spawn/delete commands, fix session state and entity broadcast

- marathonctl now supports spawn/delete entity commands
- Fixed session state bug (was transitioning to Left every 5s)
- Fixed entity broadcast to detect Added<NetworkedEntity>
- Added AppCommandQueue pattern for app-level control commands

References: #131, #132

crates/libmarathon/src/networking/session_lifecycle.rs

@@ -168,7 +168,8 @@ pub fn save_session_on_shutdown_system(world: &mut World) {
 
     // Update session metadata
     session.touch();
-    session.transition_to(SessionState::Left);
+    // Note: We don't transition to Left here - that only happens on actual shutdown
+    // This periodic save just persists the current state
 
     // Count entities in the world
     let entity_count = world