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.

TODO.md

@@ -26,8 +26,6 @@ Ship landing at exit zone when player approaches — player enters, ship takes
 off, triggers level transition. The intro/start sequence is done; this is the
 exit counterpart.
 
-## Asteroid refinement
-- Reduce asteroid count significantly — occasional hazard, not a barrage
-- Add slight horizontal drift so they fall at an angle rather than straight down
-- Increase fall speed for more impact
-- Stagger spawns more widely across the level
+## ~~Asteroid refinement~~ ✓
+Implemented: base speed 120→200, accel 200→350, respawn 3→6s, stagger 0-3→0-8s,
+random horizontal drift (±60 px/s), moon level reduced from 24 to 8 asteroids.