Add tier management system with JSON loading; refactor PopulationVisualizer for category handling

Scripts/Core/TierLoader.cs

@@ -0,0 +1,42 @@
+using System.Collections.Generic;
+using System.Linq;
+using Godot;
+using Newtonsoft.Json;
+
+namespace ParasiticGod.Scripts.Core;
+
+public static class TierLoader
+{
+    public static List<TierDefinition> LoadTiersFromFile(string filePath)
+    {
+        var loadedTiers = new List<TierDefinition>();
+        
+        var fileContent = FileAccess.GetFileAsString(filePath);
+        if (string.IsNullOrEmpty(fileContent))
+        {
+            GD.PushError($"Failed to read tier file or file is empty: {filePath}");
+            return loadedTiers;
+        }
+
+        var tierListDto = JsonConvert.DeserializeObject<TierListDto>(fileContent);
+        if (tierListDto?.Tiers == null)
+        {
+            GD.PushError($"Failed to deserialize tier list JSON or 'Tiers' array is missing: {filePath}");
+            return loadedTiers;
+        }
+
+        foreach (var dto in tierListDto.Tiers)
+        {
+            var tierDef = new TierDefinition
+            {
+                Threshold = dto.Threshold,
+                TierEnum = dto.TierEnum,
+                Scene = GD.Load<PackedScene>(dto.ScenePath) 
+            };
+            loadedTiers.Add(tierDef);
+        }
+
+        GD.Print($"Loaded {loadedTiers.Count} follower tiers from {filePath}");
+        return loadedTiers.OrderBy(t => t.Threshold).ToList();
+    }
+}