added code from unity

Runtime/PopupElementType.cs

@@ -0,0 +1,74 @@
+using System;
+using UnityEngine;
+
+namespace Jovian.PopupSystem {
+    /// <summary>
+    /// Value type identifying a popup element prefab. Compared by string ID using ordinal
+    /// comparison. Built-in types cover common popup elements. Define custom types as static
+    /// fields or create instances for game-specific elements and variants.
+    /// </summary>
+    [Serializable]
+    public struct PopupElementType : IEquatable<PopupElementType> {
+        [SerializeField] private string id;
+
+        /// <summary>The string identifier for this element type.</summary>
+        public string Id => id;
+
+        public PopupElementType(string id) {
+            this.id = id;
+        }
+
+        // --- Built-in types ---
+
+        /// <summary>Bold header text element.</summary>
+        public static readonly PopupElementType Header = new("header");
+
+        /// <summary>Body text element.</summary>
+        public static readonly PopupElementType Text = new("text");
+
+        /// <summary>Label + value stat row element.</summary>
+        public static readonly PopupElementType LabelValueText = new("label_value_text");
+
+        /// <summary>Image/icon element.</summary>
+        public static readonly PopupElementType Image = new("image");
+
+        /// <summary>Horizontal separator line.</summary>
+        public static readonly PopupElementType Separator = new("separator");
+
+        // --- Variant helper ---
+
+        /// <summary>
+        /// Creates a variant of this element type by appending a suffix.
+        /// e.g. <c>PopupElementType.Header.Variant("gold")</c> produces <c>"header_gold"</c>.
+        /// </summary>
+        public PopupElementType Variant(string variant) {
+            return new PopupElementType($"{id}_{variant}");
+        }
+
+        // --- Equality ---
+
+        public bool Equals(PopupElementType other) {
+            return string.Equals(id, other.id, StringComparison.Ordinal);
+        }
+
+        public override bool Equals(object obj) {
+            return obj is PopupElementType other && Equals(other);
+        }
+
+        public override int GetHashCode() {
+            return id != null ? id.GetHashCode() : 0;
+        }
+
+        public override string ToString() {
+            return id ?? string.Empty;
+        }
+
+        public static bool operator ==(PopupElementType left, PopupElementType right) {
+            return left.Equals(right);
+        }
+
+        public static bool operator !=(PopupElementType left, PopupElementType right) {
+            return !left.Equals(right);
+        }
+    }
+}