Spawn an exit ship when the player approaches an exit zone. The ship
flies in, lands near the exit, and waits for the player to board.
On boarding the player is deactivated, the ship takes off, and the
level transition fires after departure.
Distance-based sound effects with stereo panning for explosions, impacts,
and pickups. Spacecraft entity with full state machine (fly-in, land, take
off, fly-out), engine/synth sound loops, thruster particles, and PNG
spritesheet. Moon level intro defers player spawn until ship lands.
Also untrack build/ objects that were committed by mistake.
