Spawn just one camera (it's messing with egui)

The egui debug inspector disappears when switching scenes, which seems
to be related to the camera change. I'm not going to dig into why
exactly this is happening. I'll just create one camera and keep it.

src/lib.rs

@@ -14,10 +14,6 @@ pub struct AsteroidPlugin;
 impl Plugin for AsteroidPlugin {
     fn build(&self, app: &mut App) {
         app.add_plugins(title_screen::GameMenuPlugin)
-            .add_systems(
-                OnEnter(GameState::Playing),
-                (spawn_camera, spawn_player, spawn_ui),
-            )
             .insert_resource(ClearColor(BACKGROUND_COLOR))
             .insert_resource(WorldSize {
                 width: WINDOW_SIZE.x,
@@ -26,6 +22,11 @@ impl Plugin for AsteroidPlugin {
             .insert_resource(Lives(3))
             .register_type::<Lives>()
             .insert_resource(Score(0))
+            .add_systems(Startup, spawn_camera)
+            .add_systems(
+                OnEnter(GameState::Playing),
+                (spawn_player, spawn_ui),
+            )
             .add_systems(
                 FixedUpdate,
                 (input_ship_thruster, input_ship_rotation, wrap_entities)
@@ -41,7 +42,7 @@ impl Plugin for AsteroidPlugin {
 }
 
 #[derive(Clone, Debug, Eq, Hash, PartialEq, States)]
-enum GameState {
+pub enum GameState {
     TitleScreen, // Program is started. Present title screen and await user start
     GetReady,    // Short timer to let the player get ready after pressing start
     Playing,     // Player has started the game. Run the main loop
@@ -142,6 +143,9 @@ fn input_ship_thruster(
     mut query: Query<(&mut Velocity, &Rotation, &mut Children, &ThrusterColors), With<Ship>>,
     mut commands: Commands,
 ) {
+    // TODO: Maybe change for a Single<Ship>> so this only runs for the one ship
+    // buuut... that would silently do nothing if there are 0 or >1 ships, and
+    // I might want to crash on purpose in that case.
     let Ok((mut velocity, rotation, children, colors)) = query.single_mut() else {
         let count = query.iter().count();
         panic!("There should be exactly one player ship! Instead, there seems to be {count}.");
@@ -183,6 +187,10 @@ fn input_ship_rotation(
     }
 }
 
+// TODO: Combine movement integration steps into one function
+// They need to be ordered so the physics is deterministic. Bevy can enforce
+// order, but it makes more sense to cut out the extra machinery and have one
+// single function. Probably better for cache locality or whatever, too.
 /*
  Add velocity to position
 */

src/title_screen.rs

@@ -18,7 +18,6 @@ impl Plugin for GameMenuPlugin {
 struct TitleUI;
 
 fn spawn_menu(mut commands: Commands) {
-    commands.spawn((TitleUI, Camera2d));
     commands
         .spawn((
             TitleUI,