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.Server/Game/SchoolState.cs

@@ -4,7 +4,13 @@ namespace HSchool.Server.Game;
 /// Immutable copy of a school, safe to hand to request threads. The live <c>School</c> object
 /// never leaves its worker thread.
 /// </summary>
-internal sealed record SchoolState(int Id, string Name, DateTime GameTime, bool Running, byte SpeedIndex);
+internal sealed record SchoolState(
+    int Id,
+    string Name,
+    DateTime GameTime,
+    bool Running,
+    byte SpeedIndex,
+    IReadOnlyList<string> ModIds);
 
 /// <summary>Everything the main menu needs in one read.</summary>
 internal sealed record SchoolsState(int MaxSchools, IReadOnlyList<SchoolState> Schools);