Implement per-school save functionality by introducing a dedicated save directory and updating the school management system to support loading and saving school states. Revise documentation to reflect these changes, including updates to the architecture and design documents, and enhance the API for reloading schools from disk. Update tests to ensure proper functionality of the new save and reload features.

src/HSchool.Server/Game/SchoolState.cs

@@ -2,7 +2,7 @@ namespace HSchool.Server.Game;
 
 /// <summary>
 /// Immutable copy of a school, safe to hand to request threads. The live <c>School</c> object
-/// never leaves the loop thread.
+/// never leaves its worker thread.
 /// </summary>
 internal sealed record SchoolState(int Id, string Name, DateTime GameTime, bool Running, byte SpeedIndex);