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.Protocol/Messages.cs

@@ -46,6 +46,8 @@ public readonly record struct ServerSchoolGoneMessage(int SchoolId);
 /// <paramref name="ParentId"/> is empty for the yard.
 /// <paramref name="PupilSlots"/> is how many pupils can take a lesson here — summed from things
 /// on the server, not by the client.
+/// <paramref name="ActivitySubject"/> and <paramref name="ActivityClass"/> are empty when the
+/// room is free. <paramref name="Characters"/> are the people the timetable puts there right now.
 /// </summary>
 public sealed record MapSnapshotNode(
     byte Kind,
@@ -54,13 +56,19 @@ public sealed record MapSnapshotNode(
     string Name,
     ushort PupilSlots,
     IReadOnlyList<MapSnapshotItem> Items,
-    IReadOnlyList<string> Positions);
+    IReadOnlyList<string> Positions,
+    string ActivitySubject = "",
+    string ActivityClass = "",
+    IReadOnlyList<string>? Characters = null)
+{
+    public IReadOnlyList<string> Present => Characters ?? [];
+}
 
 /// <summary>One stacked thing in a room. <paramref name="Count"/> is 1–255.</summary>
 public sealed record MapSnapshotItem(string Name, byte Count);
 
 /// <summary>
-/// One school's map, labelled in the Hello locale. Sent once when that school is opened, not every tick.
-/// People and in-place activities are omitted — the client keeps those sections empty.
+/// One school's map, labelled in the Hello locale. Sent when that school is opened and again
+/// when the current lesson slot changes. Occupancy is computed from the timetable and the clock.
 /// </summary>
 public sealed record ServerMapSnapshotMessage(int SchoolId, IReadOnlyList<MapSnapshotNode> Nodes);