Add spatial audio, spacecraft entity, and level intro sequence

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.

src/engine/entity.h

@@ -27,6 +27,7 @@ typedef enum EntityType {
     ENT_POWERUP,
     ENT_DRONE,
     ENT_ASTEROID,
+    ENT_SPACECRAFT,
     ENT_TYPE_COUNT
 } EntityType;