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.

TODO.md

@@ -1,9 +1,15 @@
 # TODO
 
-## Distance-based sound effects
-Sounds (explosions, impacts, etc.) should attenuate based on distance from the
-player. Offscreen events far away should be inaudible. Add a helper that takes
-a world position and scales volume by proximity to the camera/player.
+## ~~Distance-based sound effects~~ ✓
+Implemented: `audio_set_listener()`, `audio_play_sound_at()` with linear
+attenuation and stereo panning. Used by enemy death, asteroid impact, and
+powerup pickup.
+
+## ~~Spacecraft art and entity~~ ✓
+Implemented: spacecraft PNG spritesheet (5 frames, 80x48 each), full entity
+with state machine (FLYING_IN → LANDING → LANDED → TAKEOFF → FLYING_OUT →
+DONE), engine/synth sounds, thruster particles, level intro sequence with
+deferred player spawn.
 
 ## Large map support (5000x5000)
 Audit the engine for anything that breaks or becomes slow at very large map
@@ -15,18 +21,10 @@ sizes. Key areas to check:
 - Camera bounds and coordinate overflow (float precision at large coords)
 - Level file parsing (row lines could exceed fgets buffer at 5000+ columns)
 
-## Spacecraft art and entity
-Create placeholder pixel art for the player's spacecraft (fits the 16x16 grid
-aesthetic but spans multiple cells, roughly 4-5 tiles wide by 2-3 tiles tall).
-Needs at minimum:
-- Landed/idle frame (visible near spawn on planet surface levels)
-- Landing animation (descending + touchdown, 3-4 frames)
-- Takeoff animation (liftoff + ascending, reuse landing in reverse)
-Used in two contexts:
-- **Level start**: ship flies in, lands at spawn, player exits, ship takes off
-  and leaves — serves as the level intro sequence
-- **Level exit**: ship flies in and lands when player nears the exit zone,
-  player enters, ship takes off — triggers level transition
+## Spacecraft at level exit
+Ship landing at exit zone when player approaches — player enters, ship takes
+off, triggers level transition. The intro/start sequence is done; this is the
+exit counterpart.
 
 ## Asteroid refinement
 - Reduce asteroid count significantly — occasional hazard, not a barrage