Impl the FuelGauge text update logic

This does a linear search through all existing FuelGauges, but we're not
expecting that to be a problem. There should only be zero or one of them
on screen at any moment.

It also doesn't work because the rest of the game state isn't correct...

src/widgets/fuel_gauge.rs

@@ -19,14 +19,26 @@ impl FuelGauge {
       },
       BackgroundColor(GREEN.into()),
       Text::new("Fuel: "),
-      children![TextSpan::new("<n>"),],
     )
   }
 
-  pub fn detect_fuel_change(event: Trigger<FuelChanged>) {
-    dbg!(format!(
-      "Detected fuel change on entity ID: {}",
-      event.target()
-    ));
+  /// Observer system to update the [`FuelGauge`] widget when a machine emits a
+  /// [`FuelChanged`] event.
+  pub fn detect_fuel_change(
+    event: Trigger<FuelChanged>,
+    fuel_levels: Query<&Fuel>,
+    mut gauges: Query<(&FuelGauge, &mut Text)>,
+  ) {
+    // Find the FuelGauge that references the same Entity as the event target.
+    // That gauge's `Text` is the one to update.
+    let (_, mut label) = gauges
+      .iter_mut()
+      .find(|(gauge, _label)| gauge.0 == event.target())
+      .expect("Couldn't find any fuel gauges");
+
+    let fuel = fuel_levels.get(event.target()).expect(
+      "Logic error: a `FuelChanged` event was targetted at an entity with no `Fuel` component.",
+    );
+    label.0 = format!("Fuel: {}", fuel.0);
   }
 }