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.
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