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/WorkerCommand.cs

@@ -0,0 +1,18 @@
+using HSchool.Server.Net;
+
+namespace HSchool.Server.Game;
+
+/// <summary>
+/// Work item for one school's worker. The supervisor never touches that school's <c>World</c>;
+/// it only posts these.
+/// </summary>
+internal abstract record WorkerCommand
+{
+    internal sealed record Open(GameClient Client) : WorkerCommand;
+
+    internal sealed record Close(uint PlayerId) : WorkerCommand;
+
+    internal sealed record SetRunning(bool Running) : WorkerCommand;
+
+    internal sealed record SetSpeed(byte SpeedIndex) : WorkerCommand;
+}