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/audio.h

@@ -2,6 +2,7 @@
 #define JNR_AUDIO_H
 
 #include <stdbool.h>
+#include "util/vec2.h"
 
 typedef struct Sound {
     void *chunk;    /* Mix_Chunk* */
@@ -21,4 +22,23 @@ void  audio_free_music(Music *m);
 void  audio_set_music_volume(int volume);       /* 0-128 */
 void  audio_shutdown(void);
 
+/* ── Looping sounds ────────────────────────────── */
+
+/* Play a sound on loop, returns the channel number (or -1 on failure).
+ * Use audio_stop_channel() to stop it later. */
+int   audio_play_sound_loop(Sound s, int volume);
+
+/* Stop a specific channel (from audio_play_sound_loop). No-op if ch < 0. */
+void  audio_stop_channel(int ch);
+
+/* ── Positional / spatial audio ────────────────── */
+
+/* Set the listener position (call once per frame, typically from camera center) */
+void  audio_set_listener(Vec2 pos);
+
+/* Play a sound at a world position. Volume is attenuated by distance from the
+ * listener and stereo-panned based on horizontal offset. Sounds beyond
+ * max_dist are inaudible. Pass max_dist <= 0 for a sensible default. */
+void  audio_play_sound_at(Sound s, int base_volume, Vec2 world_pos, float max_dist);
+
 #endif /* JNR_AUDIO_H */