feat(backend): GameProcess subprocess wrapper with async step/reset

Backend.Tests/GameProcessTests.cs

@@ -0,0 +1,54 @@
+using System.IO;
+using Backend;
+using Xunit;
+
+namespace Backend.Tests;
+
+public class GameProcessTests
+{
+    private static string LocateCliBinary()
+    {
+        var dir = AppContext.BaseDirectory;
+        while (dir is not null)
+        {
+            var candidate = Path.Combine(dir, "publish", "GameCli", "GameCli");
+            if (File.Exists(candidate)) return candidate;
+            var parent = Directory.GetParent(dir);
+            if (parent is null) break;
+            dir = parent.FullName;
+        }
+        throw new FileNotFoundException(
+            "publish/GameCli/GameCli not found — run `dotnet publish GameCli -c Release -o publish/GameCli`");
+    }
+
+    [Fact]
+    public async Task StartsAndReadsInitHandshake()
+    {
+        await using var proc = new GameProcess(LocateCliBinary());
+        await proc.StartAsync();
+        Assert.Equal(0.02f, proc.Dt);
+        Assert.Equal(7,    proc.ObsDim);
+        Assert.Equal(4,    proc.NActions);
+    }
+
+    [Fact]
+    public async Task StepReturnsSevenDimObservation()
+    {
+        await using var proc = new GameProcess(LocateCliBinary());
+        await proc.StartAsync();
+        var obs = await proc.StepAsync(action: 0, target: (0.5f, 0.5f));
+        Assert.Equal(7, obs.Observation.Length);
+        Assert.Equal(1, obs.Step);
+    }
+
+    [Fact]
+    public async Task ResetClearsStepCounter()
+    {
+        await using var proc = new GameProcess(LocateCliBinary());
+        await proc.StartAsync();
+        for (int i = 0; i < 3; i++)
+            await proc.StepAsync(action: 0, target: (0.5f, 0.5f));
+        var afterReset = await proc.ResetAsync(seed: 42);
+        Assert.Equal(0, afterReset.Step);
+    }
+}