Introduce background spawning with parallax effect (#86)

Co-authored-by: Michal Pikulski <michal.a.pikulski@gmail.com>
Reviewed-on: #86

Assets/Scripts/Minigames/Airplane/Data/SpawnPoolMode.cs

@@ -0,0 +1,21 @@
+namespace Minigames.Airplane.Data
+{
+    /// <summary>
+    /// Defines how multiple spawn pools are combined.
+    /// </summary>
+    public enum SpawnPoolMode
+    {
+        /// <summary>
+        /// All unlocked pools contribute to a single shared spawn stream.
+        /// Objects spawn at regular intervals considering all available pools.
+        /// </summary>
+        Together,
+        
+        /// <summary>
+        /// Each pool spawns independently with its own timing.
+        /// Multiple objects can spawn simultaneously from different pools.
+        /// </summary>
+        Exclusive
+    }
+}
+