feat(backend): add WebSocket wire message DTOs

Backend/Backend.csproj

@@ -12,4 +12,8 @@
     <PackageReference Include="Swashbuckle.AspNetCore" Version="6.6.2" />
   </ItemGroup>
 
+  <ItemGroup>
+    <InternalsVisibleTo Include="Backend.Tests" />
+  </ItemGroup>
+
 </Project>

Backend/WireMessages.cs

@@ -0,0 +1,43 @@
+using System.Text.Json.Serialization;
+
+namespace Backend;
+
+// -------- Client → Server --------
+
+/// <summary>Cursor position update from the browser.</summary>
+public sealed class CursorMessage
+{
+    [JsonPropertyName("type")] public string Type { get; set; } = "cursor";
+    [JsonPropertyName("x")]    public float X { get; set; }
+    [JsonPropertyName("y")]    public float Y { get; set; }
+}
+
+// -------- Server → Client --------
+
+/// <summary>Sent once on WebSocket open. Tells the client the world dimensions.</summary>
+public sealed class InitFrame
+{
+    [JsonPropertyName("type")]  public string Type { get; set; } = "init";
+    [JsonPropertyName("world")] public float[] World { get; set; } = new[] { 1f, 1f };
+}
+
+/// <summary>Sent every physics tick with the current ship pose and target.</summary>
+public sealed class StateFrame
+{
+    [JsonPropertyName("type")]   public string Type { get; set; } = "state";
+    [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; }
+    [JsonPropertyName("target")] public float[] Target { get; set; } = new[] { 0.5f, 0.5f };
+    [JsonPropertyName("step")]   public int Step { get; set; }
+}
+
+/// <summary>Source-gen JSON contracts.</summary>
+[JsonSourceGenerationOptions(WriteIndented = false)]
+[JsonSerializable(typeof(CursorMessage))]
+[JsonSerializable(typeof(InitFrame))]
+[JsonSerializable(typeof(StateFrame))]
+internal partial class WireJsonContext : System.Text.Json.Serialization.JsonSerializerContext
+{
+}