Add SceneInstanceReady (#9313)

# Objective

Closes #9115, replaces #9117.

## Solution

Emit event when scene is ready.

---

## Changelog

### Added

- `SceneInstanceReady` event when scene becomes ready.

crates/bevy_scene/src/lib.rs

@@ -40,6 +40,7 @@ impl Plugin for ScenePlugin {
         app.add_asset::<DynamicScene>()
             .add_asset::<Scene>()
             .init_asset_loader::<SceneLoader>()
+            .add_event::<SceneInstanceReady>()
             .init_resource::<SceneSpawner>()
             .add_systems(SpawnScene, (scene_spawner, scene_spawner_system).chain());
     }

crates/bevy_scene/src/scene_spawner.rs

@@ -2,7 +2,7 @@ use crate::{DynamicScene, Scene};
 use bevy_asset::{AssetEvent, Assets, Handle};
 use bevy_ecs::{
     entity::{Entity, EntityMap},
-    event::{Events, ManualEventReader},
+    event::{Event, Events, ManualEventReader},
     reflect::AppTypeRegistry,
     system::{Command, Resource},
     world::{Mut, World},
@@ -12,6 +12,15 @@ use bevy_utils::{tracing::error, HashMap, HashSet};
 use thiserror::Error;
 use uuid::Uuid;
 
+/// Emitted when [`crate::SceneInstance`] becomes ready to use.
+///
+/// See also [`SceneSpawner::instance_is_ready`].
+#[derive(Event)]
+pub struct SceneInstanceReady {
+    /// Entity to which the scene was spawned as a child.
+    pub parent: Entity,
+}
+
 /// Information about a scene instance.
 #[derive(Debug)]
 pub struct InstanceInfo {
@@ -285,6 +294,8 @@ impl SceneSpawner {
                             child: entity,
                         }
                         .apply(world);
+
+                        world.send_event(SceneInstanceReady { parent });
                     }
                 }
             } else {