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/mars01.lvl

@@ -11,7 +11,7 @@ GRAVITY 370
 WIND 25
 BG_COLOR 30 12 8
 PARALLAX_STYLE 5
-MUSIC assets/sounds/algardalgar.ogg
+MUSIC assets/sounds/kaffe_og_kage.ogg
 
 ENTITY spacecraft 1 14
 

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

assets/levels/mars03.lvl

@@ -0,0 +1,79 @@
+# Mars Base - Core Chamber (Boss Arena)
+# ======================================
+# Final Mars level: small arena room deep underground.
+# Heavy enemy presence — spawners, laser turrets, chargers.
+# Survive the onslaught to reach the exit. Gun pickup at start.
+# Exits to procedural station levels.
+
+TILESET assets/tiles/mars_tileset.png
+SIZE 30 23
+SPAWN 3 18
+GRAVITY 700
+BG_COLOR 15 8 5
+PARALLAX_STYLE 3
+MUSIC assets/sounds/kaffe_og_kage.ogg
+
+# Gun pickup right at spawn — the player needs it
+ENTITY powerup_gun 5 18
+
+# Spawners on upper ledges (both sides)
+ENTITY spawner 5 6
+ENTITY spawner 24 6
+
+# Laser turrets guarding the arena
+ENTITY laser_turret 1 10
+ENTITY laser_turret_track 28 10
+
+# Turrets on the ceiling
+ENTITY turret 10 2
+ENTITY turret 19 2
+
+# Chargers on the ground floor
+ENTITY charger 10 18
+ENTITY charger 20 18
+
+# Grunts
+ENTITY grunt 14 18
+ENTITY grunt 16 18
+
+# Health pickups on side ledges — reward for platforming up
+ENTITY powerup_hp 5 10
+ENTITY powerup_hp 24 10
+
+# Jetpack fuel on central platform
+ENTITY powerup_fuel 14 12
+
+EXIT 26 17 2 3 generate:station
+
+# Tile definitions (Mars tileset)
+TILEDEF 1 0 0 1
+TILEDEF 2 1 0 1
+TILEDEF 3 2 0 1
+TILEDEF 4 0 1 2
+
+# Collision layer (30 wide x 23 tall)
+# Enclosed arena: walls on all sides, upper ledges, central platform
+LAYER collision
+1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
+1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
+1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 1
+1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
+1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
+1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
+1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1
+1 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 1
+1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
+1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
+1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1
+1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
+1 0 0 0 0 0 0 0 0 0 0 0 0 4 4 4 4 0 0 0 0 0 0 0 0 0 0 0 0 1
+1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
+1 0 0 0 0 0 0 0 0 4 4 4 0 0 0 0 0 0 4 4 4 0 0 0 0 0 0 0 0 1
+1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
+1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
+1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
+1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
+1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
+1 3 1 1 3 1 1 3 1 3 1 1 1 3 1 1 3 1 3 1 1 3 1 1 3 1 1 3 1 1
+1 1 3 3 1 3 3 1 3 1 3 3 3 1 3 3 1 3 1 3 3 1 3 3 1 3 3 1 3 3
+3 1 3 3 1 3 3 1 3 1 3 3 3 1 3 3 1 3 1 3 3 1 3 3 1 3 3 1 3 3