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/game/level.h

@@ -18,6 +18,10 @@ typedef struct Level {
     /* ── Exit / transition state ─────────── */
     bool          exit_triggered;              /* player entered an exit zone  */
     char          exit_target[ASSET_PATH_MAX]; /* target level path            */
+
+    /* ── Intro sequence state ────────────── */
+    bool          has_intro_ship;              /* level has a spacecraft intro */
+    bool          player_spawned;              /* player has been spawned      */
 } Level;
 
 bool level_load(Level *level, const char *path);