Add spacecraft exit sequence at level exit zones

Spawn an exit ship when the player approaches an exit zone. The ship
flies in, lands near the exit, and waits for the player to board.
On boarding the player is deactivated, the ship takes off, and the
level transition fires after departure.

src/game/level.h

@@ -22,6 +22,11 @@ typedef struct Level {
     /* ── Intro sequence state ────────────── */
     bool          has_intro_ship;              /* level has a spacecraft intro */
     bool          player_spawned;              /* player has been spawned      */
+
+    /* ── Exit ship sequence state ────────── */
+    bool          exit_ship_spawned;           /* exit spacecraft has been spawned */
+    bool          exit_ship_boarded;           /* player has entered the ship     */
+    int           exit_zone_idx;               /* which exit zone the ship is for */
 } Level;
 
 bool level_load(Level *level, const char *path);