Update wire protocol to version 6 and enhance timetable functionality

- Bumped the wire protocol version to 6, reflecting changes in the communication structure.
- Expanded the timetable API with new endpoints for fetching and managing lesson schedules, including `GET /api/schools/{id}/timetable` and `POST /api/schools/{id}/timetable/pin`.
- Updated the protocol documentation to include detailed descriptions of the new timetable features and message structures.
- Enhanced the client-side implementation to support the new timetable functionalities, including lesson pinning and unpinning.
- Revised server-side logic to handle timetable operations and ensure proper integration with existing school management features.
- Added tests to validate the new timetable functionalities and ensure robustness in handling lesson data.

src/HSchool.Server/Game/SchoolStore.cs

@@ -1,6 +1,7 @@
 using System.Text.Json;
 using HSchool.Content;
 using HSchool.People;
+using HSchool.Schedule;
 using HSchool.Simulation;
 using Microsoft.Extensions.Options;
 
@@ -104,7 +105,8 @@ internal sealed class SchoolStore
         {
             var fileName = Path.GetFileName(path);
             if (string.Equals(fileName, IndexFileName, StringComparison.OrdinalIgnoreCase)
-                || fileName.EndsWith(".people.json", StringComparison.OrdinalIgnoreCase))
+                || fileName.EndsWith(".people.json", StringComparison.OrdinalIgnoreCase)
+                || fileName.EndsWith(".timetable.json", StringComparison.OrdinalIgnoreCase))
             {
                 continue;
             }
@@ -188,6 +190,12 @@ internal sealed class SchoolStore
         {
             File.Delete(people);
         }
+
+        var timetable = TimetablePath(id);
+        if (File.Exists(timetable))
+        {
+            File.Delete(timetable);
+        }
     }
 
     public RosterDocument? TryReadPeople(int id)
@@ -215,10 +223,38 @@ internal sealed class SchoolStore
         WriteAtomic(PeoplePath(id), document, RosterJson.Options);
     }
 
+    public Timetable? TryReadTimetable(int id)
+    {
+        var path = TimetablePath(id);
+        if (!File.Exists(path))
+        {
+            return null;
+        }
+
+        try
+        {
+            var json = File.ReadAllText(path);
+            return JsonSerializer.Deserialize<Timetable>(json, Json);
+        }
+        catch (Exception ex)
+        {
+            throw new SchoolContentUnavailableException(
+                $"School {id} timetable file could not be read.",
+                ex);
+        }
+    }
+
+    public void SaveTimetable(int id, Timetable table)
+    {
+        WriteAtomic(TimetablePath(id), table);
+    }
+
     private string SchoolPath(int id) => Path.Combine(DirectoryPath, $"{id}.json");
 
     private string PeoplePath(int id) => Path.Combine(DirectoryPath, $"{id}.people.json");
 
+    private string TimetablePath(int id) => Path.Combine(DirectoryPath, $"{id}.timetable.json");
+
     private string IndexPath() => Path.Combine(DirectoryPath, IndexFileName);
 
     private static void WriteAtomic<T>(string path, T value, JsonSerializerOptions? options = null)