Add unit tests across all game modules (42 tests)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/enemy.rs

@@ -32,3 +32,27 @@ impl Enemy {
         self.x + self.width
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn scroll_moves_enemy_left() {
+        let mut e = Enemy::new(200.0, 400.0, 40.0, 40.0);
+        e.scroll(200.0, 0.5);
+        assert!((e.x - 100.0).abs() < 1e-4);
+    }
+
+    #[test]
+    fn off_screen_when_right_edge_is_negative() {
+        let e = Enemy::new(-50.0, 400.0, 40.0, 40.0);
+        assert!(e.is_off_screen());
+    }
+
+    #[test]
+    fn not_off_screen_when_partially_visible() {
+        let e = Enemy::new(-20.0, 400.0, 40.0, 40.0);
+        assert!(!e.is_off_screen());
+    }
+}