Give packs an identity so create can refuse missing deps and load in a stable order.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/HSchool.Content/PackDependencyException.cs

@@ -0,0 +1,32 @@
+namespace HSchool.Content;
+
+/// <summary>
+/// Selected packs cannot be ordered: a required pack is missing, or they require each other.
+/// Distinct from <see cref="ContentLoadException"/> so the create dialog can name the pack.
+/// </summary>
+public sealed class PackDependencyException : Exception
+{
+    public const string MissingCode = "missing-mod";
+
+    public const string CycleCode = "mod-cycle";
+
+    private PackDependencyException(string message, string code, string? missingPackId)
+        : base(message)
+    {
+        Code = code;
+        MissingPackId = missingPackId;
+    }
+
+    public string Code { get; }
+
+    public string? MissingPackId { get; }
+
+    public static PackDependencyException Missing(string missingPackId, string requiredBy) =>
+        new(
+            $"Pack '{requiredBy}' requires '{missingPackId}', which was not selected.",
+            MissingCode,
+            missingPackId);
+
+    public static PackDependencyException Cycle() =>
+        new("Selected packs have a cyclic dependency.", CycleCode, missingPackId: null);
+}