Use hook on FuelGauge to show text on first spawn

There is an on-add hook that can be used to maintain invariants. I'll
use it to ensure the label text always gets the current fuel value upon
spawning the widget, regardless of whether or not the caller remembers
to set it up themselves.

src/widgets/fuel_gauge.rs

@@ -1,9 +1,15 @@
-use bevy::{color::palettes::css::*, prelude::*, ui::Val::*};
+use bevy::{
+  color::palettes::css::*,
+  ecs::{component::HookContext, world::DeferredWorld},
+  prelude::*,
+  ui::Val::*,
+};
 
 use crate::game::consumables::{Fuel, FuelChanged};
 
 #[derive(Component)]
 #[require(Label)]
+#[component(on_add = FuelGauge::refresh_label)]
 pub struct FuelGauge(Entity);
 
 impl FuelGauge {
@@ -41,4 +47,23 @@ impl FuelGauge {
     );
     label.0 = format!("Fuel: {}", fuel.0);
   }
+
+  fn refresh_label(mut world: DeferredWorld, ctx: HookContext) {
+    // Get the machine ID from the FuelGauge component.
+    let Some(machine_id) = world.get::<FuelGauge>(ctx.entity).map(|fg| fg.0) else {
+      panic!("Couldn't get a FuelGauge during it's on-add hook run!");
+    };
+    // and fuel level
+    let Some(fuel) = world.get::<Fuel>(machine_id).map(|lvl| lvl.0) else {
+      // TODO: Maybe don't panic and just emit a warning.
+      panic!("Couldn't get a Fuel for a machine during a FuelGauge on-add hook.");
+    };
+
+    // Get an excl ref to the Text component so we can redraw its text
+    let Some(mut label) = world.get_mut::<Text>(ctx.entity) else {
+      panic!()
+    };
+    
+    label.0 = format!("Fuel: {}", fuel);
+  }
 }