feat(gamecli): implement pure Physics.Step with unit tests

GameCli.Tests/PhysicsTests.cs

@@ -0,0 +1,64 @@
+using GameCli;
+using Xunit;
+
+namespace GameCli.Tests;
+
+public class PhysicsTests
+{
+    [Fact]
+    public void Noop_UnderGravity_ShipFallsDownward()
+    {
+        var start = new ShipState(0.5f, 0.5f, 0f, 0f, 0f, 0f);
+        var next = Physics.Step(start, action: 0);
+        Assert.True(next.VY > 0, $"expected VY > 0 (gravity down), got {next.VY}");
+        Assert.True(next.Y > start.Y, $"expected Y to increase (fall), got {next.Y}");
+        Assert.Equal(start.X, next.X, precision: 5);
+        Assert.Equal(0f, next.Angle);
+    }
+
+    [Fact]
+    public void MainEngine_WhenUpright_CancelsThenReversesGravity()
+    {
+        // Main engine thrust > gravity, so acceleration is net upward (VY becomes negative).
+        var start = new ShipState(0.5f, 0.5f, 0f, 0f, 0f, 0f);
+        var next = Physics.Step(start, action: 2);
+        Assert.True(next.VY < 0, $"expected VY < 0 (net upward), got {next.VY}");
+    }
+
+    [Fact]
+    public void LeftThruster_AppliesNegativeTorque()
+    {
+        var start = new ShipState(0.5f, 0.5f, 0f, 0f, 0f, 0f);
+        var next = Physics.Step(start, action: 1);
+        Assert.True(next.AngularVelocity < 0,
+            $"expected angular velocity < 0, got {next.AngularVelocity}");
+    }
+
+    [Fact]
+    public void RightThruster_AppliesPositiveTorque()
+    {
+        var start = new ShipState(0.5f, 0.5f, 0f, 0f, 0f, 0f);
+        var next = Physics.Step(start, action: 3);
+        Assert.True(next.AngularVelocity > 0,
+            $"expected angular velocity > 0, got {next.AngularVelocity}");
+    }
+
+    [Fact]
+    public void MainEngine_WhenRotated90Right_ThrustsRight()
+    {
+        // Angle = +pi/2 means ship points to the +x direction (right).
+        // Main engine pushes along body-up, which is now world +x.
+        var start = new ShipState(0.5f, 0.5f, 0f, 0f, MathF.PI / 2f, 0f);
+        var next = Physics.Step(start, action: 2);
+        Assert.True(next.VX > 0, $"expected VX > 0 (thrust right), got {next.VX}");
+    }
+
+    [Fact]
+    public void Step_IsDeterministic()
+    {
+        var start = new ShipState(0.3f, 0.4f, 0.1f, -0.05f, 0.2f, 0.3f);
+        var a = Physics.Step(start, action: 2);
+        var b = Physics.Step(start, action: 2);
+        Assert.Equal(a, b);
+    }
+}