Update .gitignore to exclude TypeScript build info and add dist directory. Expand README with project overview, technology stack, prerequisites, and instructions for running and testing the application.

src/HSchool.Server/Game/GameCommand.cs

@@ -0,0 +1,20 @@
+using HSchool.Protocol;
+
+namespace HSchool.Server.Game;
+
+/// <summary>
+/// Work item handed from a connection thread to the loop thread. The simulation is
+/// single-threaded, so every mutation arrives as one of these.
+/// </summary>
+internal abstract record GameCommand
+{
+    /// <summary>
+    /// Spawns an avatar for the connection. The loop completes <see cref="EntityId"/>
+    /// with the replication id so the handler can send a Welcome frame.
+    /// </summary>
+    internal sealed record Join(uint PlayerId, TaskCompletionSource<uint> EntityId) : GameCommand;
+
+    internal sealed record Leave(uint PlayerId) : GameCommand;
+
+    internal sealed record Input(uint PlayerId, InputButtons Buttons, uint Sequence) : GameCommand;
+}