feat(gamecli): add JSON protocol DTOs with source-gen context

GameCli/GameCli.csproj

@@ -7,4 +7,8 @@
     <Nullable>enable</Nullable>
   </PropertyGroup>
 
+  <ItemGroup>
+    <InternalsVisibleTo Include="GameCli.Tests" />
+  </ItemGroup>
+
 </Project>

GameCli/Protocol.cs

@@ -0,0 +1,56 @@
+using System.Text.Json.Serialization;
+
+namespace GameCli;
+
+/// <summary>Input line from the client (per step OR a reset command).</summary>
+public sealed class StepInput
+{
+    [JsonPropertyName("action")] public int? Action { get; set; }
+    [JsonPropertyName("target")] public float[]? Target { get; set; }
+    [JsonPropertyName("cmd")]    public string? Cmd { get; set; }
+    [JsonPropertyName("seed")]   public int? Seed { get; set; }
+}
+
+/// <summary>Output line from the CLI after each step or reset.</summary>
+public sealed class StepOutput
+{
+    [JsonPropertyName("obs")]    public float[] Obs { get; set; } = System.Array.Empty<float>();
+    [JsonPropertyName("state")]  public ShipStateDto State { get; set; } = new();
+    [JsonPropertyName("reward")] public float Reward { get; set; }
+    [JsonPropertyName("done")]   public bool Done { get; set; }
+    [JsonPropertyName("step")]   public int Step { get; set; }
+}
+
+/// <summary>Renderable ship pose (subset of ShipState that the UI actually draws).</summary>
+public sealed class ShipStateDto
+{
+    [JsonPropertyName("x")]      public float X { get; set; }
+    [JsonPropertyName("y")]      public float Y { get; set; }
+    [JsonPropertyName("angle")]  public float Angle { get; set; }
+    [JsonPropertyName("engine")] public int Engine { get; set; }
+}
+
+/// <summary>Startup handshake emitted as the very first stdout line.</summary>
+public sealed class InitMessage
+{
+    [JsonPropertyName("init")] public InitPayload Init { get; set; } = new();
+}
+
+public sealed class InitPayload
+{
+    [JsonPropertyName("world")]     public float[] World { get; set; } = new[] { 1f, 1f };
+    [JsonPropertyName("dt")]        public float Dt { get; set; }
+    [JsonPropertyName("obs_dim")]   public int ObsDim { get; set; }
+    [JsonPropertyName("n_actions")] public int NActions { get; set; }
+}
+
+/// <summary>Source-generated JSON contracts for AOT-friendly, allocation-lean (de)serialization.</summary>
+[JsonSourceGenerationOptions(WriteIndented = false)]
+[JsonSerializable(typeof(StepInput))]
+[JsonSerializable(typeof(StepOutput))]
+[JsonSerializable(typeof(ShipStateDto))]
+[JsonSerializable(typeof(InitMessage))]
+[JsonSerializable(typeof(InitPayload))]
+internal partial class ProtocolJsonContext : JsonSerializerContext
+{
+}