single global observers root node (#25)

plus auto name observer macro `global_observer`

src/abilities/thrown.rs

@@ -1,7 +1,12 @@
 use super::TriggerThrow;
 use crate::{
-    GameState, billboards::Billboard, hitpoints::Hit, loading_assets::GameAssets,
-    physics_layers::GameLayer, sounds::PlaySound, utils::sprite_3d_animation::AnimationTimer,
+    GameState,
+    billboards::Billboard,
+    hitpoints::Hit,
+    loading_assets::GameAssets,
+    physics_layers::GameLayer,
+    sounds::PlaySound,
+    utils::{global_observer, sprite_3d_animation::AnimationTimer},
 };
 use avian3d::prelude::*;
 use bevy::{pbr::NotShadowCaster, prelude::*};
@@ -35,8 +40,9 @@ pub fn plugin(app: &mut App) {
     app.add_systems(OnEnter(GameState::Playing), setup);
     app.add_systems(Update, shot_collision.run_if(in_state(GameState::Playing)));
     app.add_systems(FixedUpdate, update_auto_rotation);
-    app.add_observer(on_trigger_state);
-    app.add_observer(on_explosion);
+
+    global_observer!(app, on_trigger_thrown);
+    global_observer!(app, on_explosion);
 }
 
 fn on_explosion(
@@ -75,7 +81,7 @@ fn setup(mut commands: Commands, assets: Res<GameAssets>, mut sprite_params: Spr
     });
 }
 
-fn on_trigger_state(
+fn on_trigger_thrown(
     trigger: Trigger<TriggerThrow>,
     mut commands: Commands,
     query_transform: Query<&Transform>,