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.c

@@ -2,9 +2,17 @@
 #include "config.h"
 #include <SDL2/SDL_mixer.h>
 #include <stdio.h>
+#include <math.h>
 
 static bool s_initialized = false;
 
+/* ── Listener state for spatial audio ────────────── */
+static Vec2 s_listener_pos = {0};
+static bool s_listener_set = false;
+
+/* Default max audible distance in pixels (~40 tiles) */
+#define AUDIO_DEFAULT_MAX_DIST  640.0f
+
 bool audio_init(void) {
     /* Initialize MP3 (and OGG) decoder support */
     int mix_flags = MIX_INIT_MP3 | MIX_INIT_OGG;
@@ -66,9 +74,25 @@ void audio_play_sound(Sound s, int volume) {
     int ch = Mix_PlayChannel(-1, (Mix_Chunk *)s.chunk, 0);
     if (ch >= 0) {
         Mix_Volume(ch, volume);
+        Mix_SetPanning(ch, 255, 255);  /* reset to center (no panning) */
     }
 }
 
+int audio_play_sound_loop(Sound s, int volume) {
+    if (!s_initialized || !s.chunk) return -1;
+    int ch = Mix_PlayChannel(-1, (Mix_Chunk *)s.chunk, -1); /* -1 = loop forever */
+    if (ch >= 0) {
+        Mix_Volume(ch, volume);
+        Mix_SetPanning(ch, 255, 255);  /* reset to center */
+    }
+    return ch;
+}
+
+void audio_stop_channel(int ch) {
+    if (!s_initialized || ch < 0) return;
+    Mix_HaltChannel(ch);
+}
+
 void audio_play_music(Music m, bool loop) {
     if (!s_initialized) {
         fprintf(stderr, "audio_play_music: audio not initialized\n");
@@ -99,11 +123,60 @@ void audio_set_music_volume(int volume) {
     Mix_VolumeMusic(volume);
 }
 
+/* ── Positional / spatial audio ──────────────────── */
+
+void audio_set_listener(Vec2 pos) {
+    s_listener_pos = pos;
+    s_listener_set = true;
+}
+
+void audio_play_sound_at(Sound s, int base_volume, Vec2 world_pos, float max_dist) {
+    if (!s_initialized || !s.chunk) return;
+
+    if (max_dist <= 0.0f) max_dist = AUDIO_DEFAULT_MAX_DIST;
+
+    /* Fall back to non-positional if listener was never set */
+    if (!s_listener_set) {
+        audio_play_sound(s, base_volume);
+        return;
+    }
+
+    float dx = world_pos.x - s_listener_pos.x;
+    float dy = world_pos.y - s_listener_pos.y;
+    float dist = sqrtf(dx * dx + dy * dy);
+
+    /* Inaudible beyond max distance */
+    if (dist >= max_dist) return;
+
+    /* Linear attenuation: full volume at dist=0, silent at max_dist */
+    float atten = 1.0f - (dist / max_dist);
+    int vol = (int)(base_volume * atten);
+    if (vol <= 0) return;
+    if (vol > 128) vol = 128;
+
+    int ch = Mix_PlayChannel(-1, (Mix_Chunk *)s.chunk, 0);
+    if (ch < 0) return;
+
+    Mix_Volume(ch, vol);
+
+    /* Stereo panning: center = 127, hard-left = 0, hard-right = 254.
+     * Pan linearly based on horizontal offset relative to half the
+     * viewport width (~half of SCREEN_WIDTH). */
+    float half_screen = (float)SCREEN_WIDTH * 0.5f;
+    float pan_ratio = dx / half_screen;            /* -1..+1 roughly   */
+    if (pan_ratio < -1.0f) pan_ratio = -1.0f;
+    if (pan_ratio >  1.0f) pan_ratio =  1.0f;
+    Uint8 pan_right = (Uint8)(127 + (int)(pan_ratio * 127.0f));
+    Uint8 pan_left  = 254 - pan_right;
+    Mix_SetPanning(ch, pan_left, pan_right);
+}
+
 void audio_shutdown(void) {
     if (!s_initialized) return;
     Mix_HaltMusic();
     Mix_CloseAudio();
     Mix_Quit();
     s_initialized = false;
+    s_listener_set = false;
     printf("Audio shut down.\n");
 }

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 */

src/engine/entity.h

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