feat(backend): PolicyRunner ONNX inference wrapper

Backend.Tests/PolicyRunnerTests.cs

@@ -0,0 +1,55 @@
+using System.IO;
+using Backend;
+using Xunit;
+
+namespace Backend.Tests;
+
+public class PolicyRunnerTests
+{
+    private static string LocateOnnxModel()
+    {
+        var dir = AppContext.BaseDirectory;
+        while (dir is not null)
+        {
+            var candidate = Path.Combine(dir, "models", "ppo_lander.onnx");
+            if (File.Exists(candidate)) return candidate;
+            var parent = Directory.GetParent(dir);
+            if (parent is null) break;
+            dir = parent.FullName;
+        }
+        throw new FileNotFoundException(
+            "models/ppo_lander.onnx not found — run `python Training/export_onnx.py ...`");
+    }
+
+    [Fact]
+    public void LoadsWithoutError()
+    {
+        using var runner = new PolicyRunner(LocateOnnxModel());
+    }
+
+    [Fact]
+    public void SelectAction_ReturnsValidActionForZeroObs()
+    {
+        using var runner = new PolicyRunner(LocateOnnxModel());
+        var obs = new float[7];  // all zeros
+        int action = runner.SelectAction(obs);
+        Assert.InRange(action, 0, 3);
+    }
+
+    [Fact]
+    public void SelectAction_IsDeterministicForSameInput()
+    {
+        using var runner = new PolicyRunner(LocateOnnxModel());
+        var obs = new float[] { 0.1f, -0.2f, 0.05f, 0.0f, 0.0f, 1.0f, 0.0f };
+        int a1 = runner.SelectAction(obs);
+        int a2 = runner.SelectAction(obs);
+        Assert.Equal(a1, a2);
+    }
+
+    [Fact]
+    public void SelectAction_ThrowsIfObsLengthWrong()
+    {
+        using var runner = new PolicyRunner(LocateOnnxModel());
+        Assert.Throws<ArgumentException>(() => runner.SelectAction(new float[3]));
+    }
+}