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/spacecraft.h

@@ -0,0 +1,70 @@
+#ifndef JNR_SPACECRAFT_H
+#define JNR_SPACECRAFT_H
+
+#include "engine/entity.h"
+#include "engine/animation.h"
+
+/* ═══════════════════════════════════════════════════
+ *  Spacecraft
+ *
+ *  The player's ship. Appears at level start (fly-in
+ *  and landing sequence) and at exit zones (landing
+ *  and fly-out when the player reaches the exit).
+ *
+ *  Spritesheet: assets/sprites/spacecraft.png
+ *  Layout: 5 frames at 80x48, horizontal strip
+ *    0 = landed/idle
+ *    1 = landing frame 1 (high, bright thrusters)
+ *    2 = landing frame 2 (mid, medium thrusters)
+ *    3 = landing frame 3 (low, dim thrusters)
+ *    4 = landing frame 4 (touchdown)
+ * ═══════════════════════════════════════════════════ */
+
+/* Source sprite dimensions (pixels in PNG) */
+#define SPACECRAFT_SRC_W   80
+#define SPACECRAFT_SRC_H   48
+
+/* Render scale — drawn larger than the source art */
+#define SPACECRAFT_SCALE   2.0f
+
+/* Rendered dimensions (used for hitbox/positioning) */
+#define SPACECRAFT_WIDTH   ((int)(SPACECRAFT_SRC_W * SPACECRAFT_SCALE))
+#define SPACECRAFT_HEIGHT  ((int)(SPACECRAFT_SRC_H * SPACECRAFT_SCALE))
+
+/* State machine */
+typedef enum SpacecraftState {
+    SC_FLYING_IN,      /* approaching from off-screen        */
+    SC_LANDING,        /* playing landing animation          */
+    SC_LANDED,         /* on the ground, idle                */
+    SC_TAKEOFF,        /* playing takeoff animation (reverse landing) */
+    SC_FLYING_OUT,     /* departing off-screen               */
+    SC_DONE            /* sequence complete, can be removed  */
+} SpacecraftState;
+
+typedef struct SpacecraftData {
+    SpacecraftState state;
+    float           state_timer;      /* time spent in current state   */
+    Vec2            target_pos;       /* where to land (ground level)  */
+    Vec2            start_pos;        /* fly-in origin (off-screen)    */
+    float           fly_speed;        /* pixels/s during fly in/out    */
+    bool            is_exit_ship;     /* true = exit ship, false = intro ship */
+    int             engine_channel;   /* SDL_mixer channel for engine loop, -1 = none */
+} SpacecraftData;
+
+/* Register the spacecraft entity type */
+void spacecraft_register(EntityManager *em);
+
+/* Spawn a spacecraft that will fly in and land at the given position.
+ * The position is the bottom-center of where the ship should land. */
+Entity *spacecraft_spawn(EntityManager *em, Vec2 land_pos);
+
+/* Trigger takeoff on an existing landed spacecraft */
+void spacecraft_takeoff(Entity *ship);
+
+/* Check if the spacecraft has finished its full sequence */
+bool spacecraft_is_done(const Entity *ship);
+
+/* Check if the spacecraft is in the landed state */
+bool spacecraft_is_landed(const Entity *ship);
+
+#endif /* JNR_SPACECRAFT_H */