feat: add scrolling starfield background

Add 150 stars with randomized positions, sizes, speeds, and brightness.
Stars scroll downward with parallax depth effect and wrap around at
screen edges. Also fixes Bevy 0.13 API issues in game_state.rs
(KeyCode::R → KeyR, Input → ButtonInput).

src/starfield.rs

@@ -0,0 +1,48 @@
+use bevy::prelude::*;
+
+use crate::components::Star;
+use crate::constants::{
+    STAR_COUNT, STAR_MAX_SIZE, STAR_MAX_SPEED, STAR_MIN_SIZE, STAR_MIN_SPEED,
+    STAR_Z_DEPTH, WINDOW_HEIGHT, WINDOW_WIDTH,
+};
+
+pub fn spawn_starfield(commands: &mut Commands) {
+    for _ in 0..STAR_COUNT {
+        let size = fastrand::f32() * (STAR_MAX_SIZE - STAR_MIN_SIZE) + STAR_MIN_SIZE;
+        let speed = fastrand::f32() * (STAR_MAX_SPEED - STAR_MIN_SPEED) + STAR_MIN_SPEED;
+        let brightness = fastrand::f32() * 0.5 + 0.5; // 0.5 to 1.0
+
+        commands.spawn((
+            SpriteBundle {
+                sprite: Sprite {
+                    color: Color::rgb(brightness, brightness, brightness),
+                    custom_size: Some(Vec2::new(size, size)),
+                    ..default()
+                },
+                transform: Transform::from_translation(Vec3::new(
+                    fastrand::f32() * WINDOW_WIDTH - WINDOW_WIDTH / 2.0,
+                    fastrand::f32() * WINDOW_HEIGHT - WINDOW_HEIGHT / 2.0,
+                    STAR_Z_DEPTH,
+                )),
+                ..default()
+            },
+            Star { speed },
+        ));
+    }
+}
+
+pub fn scroll_starfield(mut star_query: Query<(&mut Transform, &Star)>, time: Res<Time>) {
+    let dt = time.delta_seconds();
+    let half_height = WINDOW_HEIGHT / 2.0;
+    let half_width = WINDOW_WIDTH / 2.0;
+
+    for (mut transform, star) in star_query.iter_mut() {
+        let parallax = -transform.translation.y * 0.01;
+        transform.translation.y -= (star.speed + parallax) * dt;
+
+        if transform.translation.y < -half_height {
+            transform.translation.y = half_height;
+            transform.translation.x = fastrand::f32() * WINDOW_WIDTH - half_width;
+        }
+    }
+}