Refine asteroids: faster, sparser, with horizontal drift

Increase base fall speed (120→200), acceleration (200→350), and respawn
delay (3→6s). Add random horizontal drift (±60 px/s) so asteroids fall
at angles. Widen initial stagger (0-3→0-8s). Reduce moon level from 24
asteroids to 8, spread evenly across the level.

src/game/hazards.h

@@ -95,13 +95,16 @@ Entity *force_field_spawn(EntityManager *em, Vec2 pos);
 
 #define ASTEROID_WIDTH     14
 #define ASTEROID_HEIGHT    14
-#define ASTEROID_FALL_SPEED 120.0f   /* base fall speed (px/s)       */
+#define ASTEROID_FALL_SPEED 200.0f   /* base fall speed (px/s)       */
+#define ASTEROID_ACCEL     350.0f    /* fall acceleration (px/s^2)   */
+#define ASTEROID_DRIFT_MAX 60.0f     /* max horizontal drift (px/s)  */
 #define ASTEROID_DAMAGE    1
-#define ASTEROID_RESPAWN   3.0f      /* seconds before respawning    */
+#define ASTEROID_RESPAWN   6.0f      /* seconds before respawning    */
 
 typedef struct AsteroidData {
     Vec2  spawn_pos;                 /* original position (for reset)*/
     float fall_speed;                /* current fall velocity        */
+    float drift_x;                   /* horizontal velocity (px/s)   */
     float respawn_timer;             /* countdown while inactive     */
     bool  falling;                   /* true = falling, false = wait */
     float trail_timer;               /* particle trail interval      */