Add Mars Base generator, mars03 boss arena, and Mars campaign polish

Dedicated Mars Base generator with 6 segment types (entry, shaft, corridor,
turret hall, hive, arena), depth-scaled difficulty, and exit to boss arena
after 2 generated levels. Mars themes integrated into all 7 generic segment
generators. Full level chain: moon03 → mars01 → mars02 → mars_base (×2) →
mars03 → station.

Also: jetpack fuel pickup preserves recharge progress, depth counters reset
on game loop, NULL checks on entity spawns, charger charge timeout, bg
decoration in Mars Base generator.

assets/levels/mars02.lvl

@@ -11,7 +11,7 @@ SPAWN 3 7
 GRAVITY 700
 BG_COLOR 20 10 6
 PARALLAX_STYLE 5
-MUSIC assets/sounds/algardalgar.ogg
+MUSIC assets/sounds/kaffe_og_kage.ogg
 
 ENTITY spacecraft 1 3
 
@@ -42,7 +42,7 @@ ENTITY powerup_hp 24 34
 ENTITY powerup_gun 12 42
 
 # Exit at bottom right — victory (end of Mars campaign)
-EXIT 36 40 2 3 generate:station
+EXIT 36 40 2 3 generate:mars_base
 
 # Tile definitions (Mars tileset)
 TILEDEF 1 0 0 1