single global observers root node (#25)

plus auto name observer macro `global_observer`

src/abilities/mod.rs

@@ -4,6 +4,7 @@ mod thrown;
 use crate::{
     GameState,
     aim::AimTarget,
+    global_observer,
     heads::ActiveHeads,
     hitpoints::Hit,
     physics_layers::GameLayer,
@@ -74,7 +75,7 @@ pub fn plugin(app: &mut App) {
 
     app.add_systems(Update, enemy_hit.run_if(in_state(GameState::Playing)));
 
-    app.add_observer(on_trigger_state);
+    global_observer!(app, on_trigger_state);
 }
 
 fn enemy_hit(