Populate minigame with obstacles and monster spawns (#5)

- Simulated "fake" physics and collisions
- Object pooling for tiles, obstacles and monster spawns
- Base monster scoring with proximity triggers and depth multiplier

Co-authored-by: AlexanderT <alexander@foolhardyhorizons.com>
Co-authored-by: Michal Pikulski <michal.a.pikulski@gmail.com>
Reviewed-on: #5

Assets/Scripts/Minigames/DivingForPictures/ObstacleCollision.cs

@@ -0,0 +1,44 @@
+using UnityEngine;
+
+namespace Minigames.DivingForPictures
+{
+    /// <summary>
+    /// Collision behavior that handles damage from mobile obstacles.
+    /// Uses trigger-based collision detection with shared immunity state.
+    /// </summary>
+    public class ObstacleCollision : PlayerCollisionBehavior
+    {
+        protected override void HandleCollisionResponse(Collider2D obstacle)
+        {
+            // Mark the obstacle as having dealt damage to prevent multiple hits
+            FloatingObstacle obstacleComponent = obstacle.GetComponent<FloatingObstacle>();
+            if (obstacleComponent != null)
+            {
+                obstacleComponent.MarkDamageDealt();
+            }
+            
+            Debug.Log($"[ObstacleCollision] Player hit by obstacle {obstacle.gameObject.name}");
+        }
+        
+        /// <summary>
+        /// Override to prevent input blocking during damage immunity
+        /// Since obstacles pass through the player, we don't want to block input
+        /// </summary>
+        protected override void OnImmunityStart()
+        {
+            Debug.Log($"[ObstacleCollision] Damage immunity started for {damageImmunityDuration} seconds");
+            
+            // Don't block input for obstacle damage - let player keep moving
+            // The shared immunity system will handle the collision prevention
+        }
+        
+        /// <summary>
+        /// Override to handle immunity end
+        /// </summary>
+        protected override void OnImmunityEnd()
+        {
+            Debug.Log($"[ObstacleCollision] Damage immunity ended");
+            // No special handling needed - shared immunity system handles collider re-enabling
+        }
+    }
+}